Exhibit 99.(a)(1)

THIS DOCUMENT IS IMPORTANT AND REQUIRES YOUR IMMEDIATE ATTENTION. If you are in any doubt as to the action you should take, you are recommended to seek your own personal financial advice from your stockbroker, bank manager, solicitor, accountant or other independent financial adviser authorised under the Financial Services and Markets Act 2000 if you are resident in the United Kingdom or, if not, from another appropriately authorised independent financial adviser.

This document should be read in conjunction with the accompanying Acceptance Forms, which form part of this document.

If you have sold or otherwise transferred all of your Celltech Shares or Celltech ADSs (other than pursuant to the Offer), please send this document, together with the accompanying documents and the reply-paid envelope as soon as possible to the purchaser or transferee, or to the stockbroker, bank or other agent through whom the sale or transfer was effected, for transmission to the purchaser or transferee. If you have sold or otherwise transferred only part of your holding of Celltech Shares or Celltech ADSs, you should retain these documents. However, the foregoing documents must not be forwarded or transmitted in or into Australia, Belgium, Canada or Japan or any jurisdiction where to do so would constitute a violation of the relevant laws in that jurisdiction.

Recommended Cash Offer
by
Lazard & Co., Limited
on behalf of
UCB S.A.
and (in the United States) by UCB S.A. itself
for
Celltech Group plc

YOUR ATTENTION IS DRAWN TO THE LETTER FROM THE CHAIRMAN OF CELLTECH, WHICH IS SET OUT IN PART I OF THIS DOCUMENT AND WHICH CONTAINS THE RECOMMENDATION OF THE CELLTECH BOARD TO ACCEPT THE OFFER.

IF YOU ARE A HOLDER OF CELLTECH SHARES AND WISH TO ACCEPT THE OFFER, THE ACCOMPANYING FORM OF ACCEPTANCE SHOULD BE COMPLETED, SIGNED AND RETURNED, WHETHER OR NOT YOUR CELLTECH SHARES ARE HELD IN CREST, IN ACCORDANCE WITH THE INSTRUCTIONS PRINTED THEREON, AS SOON AS POSSIBLE AND, IN ANY EVENT, SO AS TO BE RECEIVED BY CAPITA IRG PLC, CORPORATE ACTIONS, PO BOX 166, THE REGISTRY, 34 BECKENHAM ROAD, BECKENHAM, KENT BR3 4TH NOT LATER THAN 3.00 P.M. (LONDON TIME), 10.00 A.M. (NEW YORK CITY TIME), ON 17 JUNE 2004.

THE PROCEDURE FOR ACCEPTING THE OFFER IS SET OUT IN PARAGRAPH 18 OF THE LETTER FROM LAZARD SET OUT IN PART II OF THIS DOCUMENT AND IN THE FORM OF ACCEPTANCE.

IF YOU ARE A HOLDER OF CELLTECH ADSs AND WISH TO ACCEPT THE OFFER, YOU SHOULD FOLLOW THE INSTRUCTIONS SET OUT IN PARAGRAPH 18(c) OF THE LETTER FROM LAZARD SET OUT IN PART II OF THIS DOCUMENT AND IN THE LETTER OF TRANSMITTAL.

        Lazard is acting for UCB and no one else in connection with the Offer and will not be responsible to anyone other than UCB for providing the protections afforded to clients of Lazard or for providing advice in relation to the Offer or the contents of this document.

        Each of Morgan Stanley and JPMorgan is acting for Celltech and no one else in connection with the Offer and will not be responsible to anyone other than Celltech for providing the protections afforded to clients of Morgan Stanley or JPMorgan (as the case may be) or for providing advice in relation to the Offer or the contents of this document.

        The Offer in the United States is made solely by UCB and neither Lazard nor any of its affiliates is making the Offer in the United States.

        Unless otherwise determined by UCB and permitted by applicable law and regulation, the Offer (including the Loan Note Alternative) will not be made, directly or indirectly, in or into a Restricted Jurisdiction and, subject to certain exemptions, the Offer will not be capable of acceptance from or within a Restricted Jurisdiction. Accordingly, copies of this document and the Acceptance Forms and any other accompanying document are not being, and must not be, directly or indirectly, mailed, forwarded, transmitted, sent or otherwise distributed or sent in, into or from a Restricted Jurisdiction and persons receiving this document, the Acceptance Forms and any other accompanying document (including custodians, nominees and trustees) must not mail, forward, transmit, send or otherwise distribute them in, into or from such jurisdictions as doing so may invalidate any purported acceptance of the Offer. The availability of the Offer to persons who are not resident in the United Kingdom or the United States may be affected by the laws of their relevant jurisdiction. Such persons should inform themselves about and observe any applicable legal or regulatory requirements of their jurisdiction.

        The Loan Notes which may be issued pursuant to the Loan Note Alternative have not been, and will not be, listed on any stock exchange and have not been, and will not be, registered under the Securities Act or under any relevant laws of any state or other jurisdiction of the United States, nor have clearances been, nor will they be, obtained from the securities commission or similar authority of any province or territory of Canada and no prospectus has been, or will be, filed, or registration made, under any securities law of any province or territory of Canada, nor has a prospectus in relation to the Loan Notes been, nor will one be, lodged with, or registered by, the Australian Securities and Investments Commission, nor have any steps been taken, nor will any steps be taken, to enable the Loan Notes to be offered in compliance with applicable securities laws of Japan or Belgium. Accordingly, unless an exemption under relevant securities laws is available, the Loan Notes may not be offered, sold, re-sold or delivered, directly or indirectly, in, into or from the United States or a Restricted Jurisdiction in which an offer of Loan Notes would constitute a violation of relevant laws or require registration of the Loan Notes, or to or for the account or benefit of any US person or resident of a Restricted Jurisdiction. Under the terms of the Offer, holders of Celltech Securities who are US persons or otherwise located in the United States are not eligible to receive Loan Notes.

        Any person (including nominees, trustees and custodians) who would, or otherwise intends to, or may have a legal or contractual obligation to, forward this document and/or any related document to any jurisdiction outside the United Kingdom or the United States, should read paragraph 6 of Part B of Appendix I to this document before taking any action.

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TO ACCEPT THE OFFER

1.

Complete the Form of Acceptance in accordance with paragraph 18 of the letter from Lazard set out in Part II of this document (see pages 9 to 25). A step-by-step guide to completing the Form of Acceptance has been sent to you with this document.

2.

Return as soon as possible the completed Form of Acceptance (along with any appropriate documents of title, such as your share certificate(s)) using the enclosed reply-paid envelope and, in any event, so as to be received by Capita IRG Plc, Corporate Actions, PO Box 166, The Registry, 34 Beckenham Road, Beckenham, Kent, BR3 4TH, no later than 3.00 p.m. (London time), 10.00 a.m. (New York City time), on 17 June 2004.

3.

If you hold your Celltech Shares in CREST, you should follow the additional procedures set out in paragraph 18(b) of the letter from Lazard set out in Part II of this document (see pages 9 to 25).

SHAREHOLDER HELPLINE 0870 162 3118 (calls charged at national rate)                       
(800) 261 1054 (toll free, if telephoning in the US)
+44 208 639 2157 (if telephoning from other countries)

Open Monday to Friday, 9 a.m. to 5 p.m. (London time)

        For legal reasons, the Shareholder Helpline will only be able to provide information contained in this document and the Form(s) of Acceptance and will be unable to give advice on the merits of the Offer or to provide financial advice.

Acceptances of the Offer must be received by 3.00 p.m. (London time), 10.00 a.m. (New York City time) on 17 June 2004.

IF YOU ARE A HOLDER OF CELLTECH ADSs, TO ACCEPT THE OFFER:

1.

complete the Letter of Transmittal in accordance with paragraph 18(c) of the letter from Lazard; and

2.

return as soon as possible the completed Letter of Transmittal (along with any appropriate documents of title, such as your Celltech ADRs) to the Tender Agent, using the enclosed reply-paid envelope.

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Offer into the United States

If you are a resident of the United States, please read the following:

        The Offer is being made for securities of a UK company and, while the Offer is subject to UK and US disclosure requirements, US investors should be aware that this Offer Document has been prepared in accordance with a UK format and style, which differs from the US format and style. In particular, the appendices to this document contain information concerning the Offer required by UK and US disclosure requirements which may be material and which has not been summarised elsewhere in this document. In addition, the financial statements of UCB reproduced in this document have been prepared in accordance with generally accepted accounting principles in Belgium and the financial statements of Celltech reproduced in this document have been prepared in accordance with generally accepted accounting principles in the United Kingdom and thus neither set of financial statements may be comparable to financial statements of US companies or companies whose financial statements are prepared in accordance with generally accepted accounting principles in the United States. In addition, the settlement procedure with respect to the Offer will comply with the rules of the City Code, which differ from US domestic tender offer procedures in certain material respects, particularly with regard to the date of payment of consideration.

        Celltech is organised under the laws of England and Wales. UCB is organised under the laws of Belgium. Some or all of the officers and directors of Celltech and UCB, respectively, are residents of countries other than the United States. In addition, a significant portion of the assets of Celltech and UCB are located outside the United States. As a result, it may be difficult for US shareholders of Celltech to effect service of process within the United States upon Celltech or UCB or their respective officers or directors or to enforce against them a judgement of a US court predicated upon the federal or state securities laws of the United States.

        As part of the Offer, Celltech Shareholders who are eligible to do so may elect for the Loan Note Alternative. The effect of electing for the Loan Note Alternative will be to allow eligible Celltech Shareholders resident in the United Kingdom to defer any taxable gain arising on a disposal of their Celltech Shares until such time as the Loan Notes are transferred or redeemed. Such tax treatment is not available for the Loan Note Alternative under US federal income tax laws and the Loan Note Alternative is not available to Celltech Shareholders who are US persons or otherwise in the United States. The Loan Note Alternative is also not available to holders of Celltech ADSs.

        In accordance with the City Code, normal UK practice and Rule 14e-5 under the Exchange Act ("Rule 14e-5"), Lazard, Morgan Stanley and JPMorgan and/or their respective affiliates will continue to act as connected exempt market makers or connected exempt principal traders in Celltech Shares on the London Stock Exchange. Information regarding such activities which is required to be made public in the United Kingdom pursuant to the City Code is reported to a Regulatory Information Service of the UK Listing Authority. This information will also be made available to US holders of Celltech Shares and Celltech ADSs, if such holders contact the UK or US Information Agent on the Helpline.

        In addition, in accordance with normal UK practice and pursuant to exemptive relief granted by the SEC from Rule 14e-5, UCB and its nominees or brokers (acting as agents) may make certain purchases of, or arrangements to purchase, Celltech Shares outside the United States during the period in which the Offer remains open for acceptance. In accordance with the requirements of Rule 14e-5 and with the exemptive relief granted by the SEC, such purchases, or arrangements to purchase, must comply with applicable UK rules, including the City Code, the rules of the UK Listing Authority and the rules of the London Stock Exchange. This information will be disclosed in the United States through amendments to UCB's Tender Offer Statement on Schedule TO on file with the SEC to the extent that such information is made public in the United Kingdom pursuant to the City Code. Free copies of the Tender Offer Statement are available on the SEC's website at http://www.sec.gov.

        This Offer Document includes "forward-looking statements" relating to the Offer, UCB and Celltech that are subject to known and unknown risks and uncertainties, many of which are outside of UCB's and Celltech's control and are difficult to predict and that may cause actual results to differ materially from any future results expressed or implied by such forward-looking statements. In this Offer Document, the words "anticipates," "believes," "estimates," "seeks," "expects," "plans," "intends" and similar expressions, as they relate to UCB or its management, are intended to identify forward-looking statements. Important factors that could cause actual results to differ materially from such expectations include, without limitation: the inability to obtain necessary regulatory approvals in the context of the Offer or to obtain

iii

them on acceptable terms; the inability to integrate successfully Celltech within UCB or to realise synergies from such integration following the acquisition; costs related to the acquisition of Celltech; the economic environment of the industries in which UCB and Celltech operate; costs associated with research and development; changes in the prospects for products in the pipeline or under development by UCB or Celltech; dependence on the existing management of UCB and Celltech; changes or uncertainties in UK or US federal or state tax laws or the administration of such laws; changes or uncertainties in the laws or regulations applicable to the markets in which UCB and Celltech operate, including those of the Federal Drug Administration in the United States; and other factors detailed in Celltech's filings with the SEC.

        All subsequent written and oral forward-looking statements attributable to UCB or Celltech or persons acting on behalf of either of them are expressly qualified in their entirety by the cautionary statements above. The forward-looking statements included herein are made only as of the date of this Offer Document. Neither UCB nor Celltech intend, or undertake any obligation, to update these forward-looking statements.

        Any person who, alone or acting together with any other person(s) pursuant to an agreement or any understanding (whether formal or informal) to acquire or control securities of Celltech, owns or controls, or becomes the owner or controller, directly or indirectly, of one per cent. or more of the issued share capital of Celltech is generally required under the provisions of Rule 8 of the City Code to notify a Regulatory Information Service and the Panel of every dealing in such securities during the Offer Period by not later than 12.00 noon (London time) on the business day following the date of the relevant transaction. Please consult your financial adviser immediately if you believe this Rule may be applicable to you.

        In the United States, the Offer is made solely by UCB, and neither Lazard nor any of its affiliates is making the Offer in the United States.

        Lazard Frères & Co. LLC is acting as dealer manager of the Offer in the United States.

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CONTENTS

v

FREQUENTLY ASKED QUESTIONS

        The following are some of the questions you, as a Celltech Shareholder and/or holder of Celltech ADSs, may have and answers to those questions. You are advised to read carefully the remainder of this document and the accompanying Form of Acceptance (in relation to Celltech Shares) or Letter of Transmittal (in relation to Celltech ADSs).

1      Who is offering to buy my shares?

The Offer is being made by UCB, a company organised under the laws of Belgium, and (outside the United States) by Lazard on its behalf. The main listing of UCB's ordinary shares is on the First Market of Euronext Brussels.

UCB is headquartered in Brussels, Belgium and is a pharmaceutical and specialty chemical company that does business through its approximately 120 subsidiaries in Europe, the Americas and Asia. UCB's businesses are focused in two sectors, UCB Pharma and Surface Specialties.

2      What are the classes and amounts of Celltech Securities sought in the Offer?

UCB is seeking to acquire all of the issued and to be issued Celltech Securities, comprised of:

•

Celltech Shares listed on the London Stock Exchange; and



•

Celltech ADSs listed on the New York Stock Exchange.

3      What will I receive in exchange for my Celltech Securities?

UCB is offering to pay:

4      How do I accept the Offer?

If you are a holder of Celltech Shares, to accept the Offer, you must deliver a completed Form of Acceptance to the Receiving Agent not later than the time and date on which the Offer expires (see question 8 below). If you hold your Celltech Shares in certificated form, you must also deliver your share certificates. If you hold your Celltech Shares in CREST, you must follow the instructions set out in paragraph 18(b) of the letter from Lazard set out in Part II of this document.

If you are a holder of Celltech ADSs, to accept the Offer, you must deliver your Celltech ADRs evidencing your Celltech ADSs, together with a completed Letter of Transmittal, to the Tender Agent not later than the time and date on which the Offer expires (see question 8 below).

If your Celltech ADSs are held in a "street name" in the United States, your nominee can tender them through the applicable book entry transfer system. In addition, in the case of Celltech ADSs, if you cannot get any document or instrument that is required to be delivered by the expiration of the Offer, you may gain some time by following the procedures for guaranteed delivery. See paragraph 18(c) of the letter from Lazard and paragraph 2 of Part C of Appendix I to this document.

5      How does the Offer compare with recent prices of Celltech Shares and Celltech ADSs?

The Offer for Celltech Shares represents a premium of:

•

27.8 per cent. to the Closing Price of 430.5 pence per Celltech Share on 17 May 2004, the last business day prior to the announcement by Celltech of the Offer;



•

26.3 per cent. to the average Closing Price of 435.6 pence per Celltech Share during the three months prior to 17 May 2004; and



•

44.7 per cent. to the average Closing Price of 380.1 pence per Celltech Share during the twelve months prior to 17 May 2004.

See paragraph 3 of Appendix V to this document for the variation in the prices of Celltech Shares and Celltech ADSs.

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6      Do the Directors of Celltech support the Offer?

Yes. The Celltech Board, which has been so advised by Morgan Stanley and JPMorgan, considers the terms of the Offer to be fair and reasonable. In providing advice to the Celltech Board, Morgan Stanley and JPMorgan have taken into account the commercial assessments of the Celltech Board. Accordingly, the Celltech Board unanimously recommends that Celltech Shareholders accept the Offer, as they and certain members of their families have irrevocably undertaken to do in respect of Celltech Shares beneficially owned and controlled by them. See the letter from the Chairman of Celltech in the next section of this document.

7      Does UCB have the financial resources to make payment?

Yes. The Offer will be financed from funds made available to UCB pursuant to financing agreements. The Offer is not conditional upon any financing arrangements. See paragraph 7 of Appendix V to this document.

8      How long do I have to accept the Offer?

You will have until 3.00 p.m. (London time), 10.00 a.m. (New York City time), on 17 June 2004 to accept the Offer or withdraw your acceptance, unless the Initial Offer Period is extended. In addition, you may accept the Offer but not withdraw your acceptance during the Subsequent Offer Period except in the limited circumstances described in paragraph 3 of Part B of Appendix I. If you are a holder of Celltech ADSs and you cannot deliver everything that is required in order to make a valid tender of Celltech ADSs by that time, you may be able to use a Guaranteed Delivery Procedure, which is described later in this document. See paragraph 18(c) of the letter from Lazard and paragraph 2 of Part C of Appendix I to this document.

9      Until what time can I withdraw my acceptance?

The Initial Offer Period for acceptances and withdrawals is the period from the date of this document until the time and date (not being before 3.00 p.m. (London time), 10.00 a.m. (New York City time)), on 17 June 2004 and not, except with the consent of the Panel, being after 1.00 p.m. (London time), 8.00 a.m. (New York City time) on 18 July 2004) on which date all the Conditions are satisfied, fulfilled or, to the extent permitted, waived or, if earlier, the time and date on which the Offer lapses.

Unless the Offer has lapsed, the Subsequent Offer Period starts as soon as the Initial Offer Period terminates. The Subsequent Offer Period must remain open for at least 14 days but it may be extended beyond that time by UCB until a further specified date or until further notice.

You can withdraw your acceptance during the Initial Offer Period but not during the Subsequent Offer Period except in the limited circumstances described in paragraph 3 of Part B of Appendix I. See paragraph 3 of Part B of Appendix I of this document.

10    Can the Offer be extended and under what circumstances?

If all of the Conditions have not been either satisfied, fulfilled or, to the extent permitted, waived by UCB by 3.00 p.m. London time, 10.00 a.m. (New York City time) on 17 June 2004, UCB may choose, but shall not be obliged, to extend the Initial Offer Period. UCB may also be required to extend the Initial Offer Period under applicable UK and US securities laws if it changes the Offer in any material respect. The Initial Offer Period for acceptances and withdrawals cannot be extended beyond 1.00 p.m. (London time), 8.00 a.m. (New York City time) on 18 July 2004 without the consent of the Panel.

Once all the Conditions have been either satisfied, fulfilled or, to the extent permitted, waived by UCB it will extend the Offer for a Subsequent Offer Period of at least 14 days. See paragraph 1 of Part B of Appendix I to this document.

11    How will I be notified if the Offer is extended?

If UCB extends the Offer, it will make a public announcement of the extension not later than 8.00 a.m. (London time) in the United Kingdom, and 8.00 a.m. (New York City time) in the United States, on the next business day after the date on which the Offer was scheduled to expire. See paragraph 1 of Part B of Appendix I to this document.

UCB will also announce by not later than 8.00 a.m. (London time) in the United Kingdom, and 8.00 a.m. (New York City time) in the United States, on the business day following the end of the

2

Initial Offer Period that there will be a Subsequent Offer Period. The Subsequent Offer Period will remain open for at least 14 days but UCB may extend it beyond that time until a further specified date or until further notice.

12    What are the most significant conditions to the Offer?

Unless it has received valid acceptances (which have not been properly withdrawn) in respect of at least 90 per cent. of the Celltech Shares (including Celltech Shares represented by Celltech ADSs) to which the Offer relates, UCB is not obliged to purchase any Celltech Shares and/or Celltech ADSs. This percentage may be reduced at the discretion of UCB, subject to certain limits. At least five US business days prior to any reduction, UCB will announce that it may do this through a press release and an advertisement in a newspaper with general circulation in the United States.

UCB is not obliged to purchase any Celltech Shares and/or Celltech ADSs unless, among other things, each of the antitrust authorities in Germany and Austria approve the transaction on terms reasonably satisfactory to UCB. In relation to the United Kingdom, the Offer is conditional on the OFT indicating either that they do not have jurisdiction over the transaction or do not intend to refer the transaction to the Competition Commission and in relation to the US, the Offer is conditional on a filing being made and the applicable waiting period under the United States Hart-Scott-Rodino Antitrust Improvements Act of 1976 having expired or been terminated.

See Part A of Appendix I to this document.

13    How do I withdraw my acceptance?

To withdraw an acceptance in relation to the Offer for Celltech Shares or Celltech ADSs, you must deliver a written notice of withdrawal with the required information to the Receiving Agent or the Tender Agent, as applicable, while you still have the right to withdraw the Celltech Shares or Celltech ADSs. See paragraph 3 of Part B of Appendix I to this document.

14    Will the Offer be followed by a compulsory acquisition?

If all of the Conditions to the Offer are either satisfied, fulfilled or, where permitted, waived and UCB has acquired 90 per cent. in nominal value of Celltech Shares (including Celltech Shares represented by Celltech ADSs) within the statutory time period, then UCB will be entitled to and intends to acquire all remaining Celltech Shares (including those represented by Celltech ADSs) pursuant to the Companies Act. Celltech Shareholders and holders of Celltech ADSs subject to the compulsory acquisition would be offered the same consideration as those Celltech Shareholders and holders of Celltech ADSs who accept the Offer. See paragraph 7(c) of Part B of Appendix I to this document.

15    If I decide not to accept, how will the Offer affect my securities?

If UCB is able to, it will acquire all Celltech Shares (including Celltech Shares represented by Celltech ADSs) for which it has not received acceptances pursuant to the compulsory acquisition provisions of the Companies Act. UCB also intends to procure the making of an application by Celltech for the removal of Celltech Shares from the Official List and for the cancellation of trading in Celltech Shares on the London Stock Exchange's market for listed securities. It is anticipated that cancellation of listing and trading will take effect no earlier than 20 Business Days after the Offer becomes or is declared unconditional in all respects. UCB also intends to procure that Celltech applies for de-listing of the Celltech ADSs from the New York Stock Exchange. Such de-listings and cancellation would significantly reduce the liquidity and marketability of any Celltech Securities not tendered in the Offer. UCB may also request that Celltech terminate the existing deposit agreement through which the ADS programme is operated. See paragraph 7(d) of Part B of Appendix I to this document.

16    Can I choose the currency of the cash that I receive?

If you accept the Offer for Celltech Shares, you will receive the price for your shares in pounds sterling.

If you accept the Offer for Celltech ADSs, you will receive the price for your ADSs in US dollars unless you specifically elect to receive it in pounds sterling.

Where you are receiving US dollars, the cash amount payable in pounds sterling to which you would otherwise be entitled pursuant to the terms of the Offer will be converted, without charge, from pounds sterling to US dollars at the exchange rate obtainable on the spot market in London on the date the cash consideration is made available by UCB to the Tender Agent for delivery in respect of

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your Celltech ADSs. The actual amount of US dollars received will depend upon the exchange rate prevailing on the day on which funds are made available to the Tender Agent by UCB.

See paragraph 20 of the letter from Lazard and paragraph 2(j) of Part C of Appendix I to this document.

17    Will I have to pay any fees or commissions?

If you are the registered owner of your Celltech Shares and/or Celltech ADSs and you accept the Offer, you will not have to pay brokerage fees or similar expenses. If you own your Celltech Shares and/or Celltech ADSs through a broker or other nominee, and your broker accepts the Offer on your behalf, your broker or nominee may charge you a fee for doing so. You should consult your broker or nominee to determine whether any charges will apply.

18    Will I be taxed on the cash that I receive?

For UK tax purposes, a UK resident holder who accepts the Offer and elects for the cash consideration (to the extent that cash consideration, rather than Loan Notes, is received in return for those Celltech Shares) will generally realise an immediate chargeable gain or allowable loss if the Offer becomes unconditional.

For US federal income tax purposes, a US Holder will generally recognise a capital gain or loss on the exchange of Celltech Shares or Celltech ADSs in an amount equal to the difference between the US Holder's tax basis in its Celltech Shares or Celltech ADSs and the offer consideration valued in US dollars. A US Holder may also recognise an exchange gain or loss due to currency fluctuations.

Further information regarding the application of both UK and US tax laws to holders of Celltech Securities who accept the Offer is set out in paragraph 16 of the letter from Lazard set out in Part II of this document.

19    Is there an alternative to cash consideration for my Celltech Securities?

As part of the Offer, Celltech Shareholders who are eligible to do so may elect to receive some or all of their consideration in Loan Notes. For UK tax purposes, the effect of electing for the Loan Note Alternative should be to allow eligible Celltech Shareholders to defer any taxable gain arising on a disposal of their Celltech Shares (to the extent that Loan Notes, rather than cash, are received in return for those Celltech Shares) until such time as the Loan Notes are transferred or redeemed. Such tax treatment is not available for the Loan Note Alternative under US federal income tax laws and the Loan Note Alternative is not available to Celltech Shareholders who are US persons or persons resident in a Restricted Jurisdiction. The Loan Note Alternative is also not available to holders of Celltech ADSs.

Further information relating to the Loan Note Alternative is set out in paragraph 3 of the letter from Lazard and Appendix II to this document.

20    Who can answer questions I might have about the Offer?

If you have any questions about procedures for acceptance of the Offer, you should contact the Helpline on one of the following numbers:

•

From the United Kingdom*: 0870 162 3118



•

From other countries: +44 208 639 2157



•

Toll free in the United States: (800) 261 1054

*

Call charged at national rate

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PART I

LETTER FROM THE CHAIRMAN OF CELLTECH

        19 May 2004

To:    Celltech Shareholders and holders of Celltech ADSs and, for information only, to participants in the Celltech Share Plans

Dear Shareholder or ADS holder,

RECOMMENDED CASH OFFER BY UCB FOR CELLTECH

1      Introduction

        The boards of UCB and Celltech announced on 18 May 2004 that they have agreed the terms of a recommended cash offer for the entire issued and to be issued share capital of Celltech.

        I am now writing to you to set out the terms of the Offer, to explain the background to and the reasons for our recommendation of the Offer and to seek your acceptance of the Offer.

2      Summary Terms of the Offer

        The Offer (on the terms and subject to the conditions set out in this document and in the Acceptance Forms) is contained in the letter from Lazard, UCB's financial adviser, set out in Part II of this document.

        Under the terms of the Offer, holders of Celltech Securities will receive:

        This represents:

        The Offer values the entire issued and to be issued share capital of Celltech at approximately £1,530 million.

        The Offer represents a premium of:

•

27.8 per cent. to the Closing Price of 430.5 pence per Celltech Share on 17 May 2004, the last business day prior to the announcement by Celltech of the Offer;

•

26.3 per cent. to the average Closing Price of 435.6 pence per Celltech Share during the three months prior to 17 May 2004; and

•

44.7 per cent. to the average Closing Price of 380.1 pence per Celltech Share during the twelve months prior to 17 May 2004.

        The Celltech Shares (including those represented by Celltech ADSs) will be acquired pursuant to the Offer fully paid and free from all liens, charges, equitable interests, encumbrances, rights of pre-emption

Celltech Group plc    208 Bath Road    Slough    Berkshire SL1 3WE    United Kingdom
Tel: +44(0) 1753 534655    Fax: +44(0) 1753 536632    www.celltechgroup.com
Registered Office as above.    Registered in England No. 2159282

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and any other rights and interests of any nature whatsoever and together with all rights now and hereafter attaching thereto, including voting rights and the right to receive and retain in full all dividends and other distributions (if any) declared, made or paid on or after the date of the Announcement.

3      Loan Note Alternative

        As an alternative to some or all of the cash consideration of 550 pence per Celltech Share which they would otherwise receive under the Offer, Celltech Shareholders (other than certain overseas shareholders) who validly accept the Offer will be able to elect to receive Loan Notes. The Loan Note Alternative will be made available on the following basis:

for every whole £1 in cash consideration £1 nominal value of Loan Notes

        A summary of the terms of the Loan Note Alternative is set out in the letter from Lazard in Part II of this document and further details of the Loan Notes are set out in Appendix II to this document.

4      Information on Celltech

        Celltech is one of the largest European-based biopharmaceutical companies, possessing significant discovery and development capabilities, a broad product pipeline, and an international pharmaceutical business, with operations in the United States and Europe. It derives revenues from the licensing of its technologies and products and the sale of pharmaceutical products through its pharmaceutical business.

        The discovery and development activities are focused on treatments for auto-immune and inflammatory disorders and oncology. Its pipeline includes candidates comprising new chemical entities and antibody-based therapeutics, in pre-clinical or clinical development and marketing licence registration. Its technology base includes a leading position in antibody engineering and extensive medicinal chemistry capabilities. Celltech has a range of discovery, development and commercialisation collaborations with leading pharmaceutical and biotechnology companies including: Abgenix, Amgen, AstraZeneca, Biogen Idec, Johnson & Johnson, Merck, NeoGenesis, Seattle Genetics and Wyeth.

        Celltech Shares are traded on the London Stock Exchange and Celltech ADSs are listed on the New York Stock Exchange. Cazenove & Co. Limited acts as broker to Celltech.

5      Celltech current trading

        Celltech announced its preliminary results for the year ended 31 December 2003 on 16 March 2004 and posted the annual report and accounts for such period to Celltech Shareholders during the week commencing 19 April 2004. For the year ended 31 December 2003, in accordance with generally accepted accounting principles in the United Kingdom, Celltech reported turnover of £353.3 million (2002: £329.6 million) with operating profit pre exceptional items and goodwill of £49.5 million (2002: £49.0 million) and net assets of £505.9 million (2002: £564.4 million). The Celltech Board considers that Celltech's business continues to perform in line with the Celltech Board's expectations.

6      Background to and reasons for recommending the Offer

        On 1 December 2003, Celltech announced that it had regained full control of CDP870 following termination of its previous collaboration agreement with Pfizer. Celltech subsequently was contacted by a number of potential partners with a view to entering into a new collaboration agreement. As announced on 31 March 2004, Celltech received expressions of interest from a range of leading pharmaceutical and biotechnology companies and over recent weeks entered detailed late stage discussions with a number of parties.

        As part of these discussions, UCB proposed terms for a collaboration agreement which the Celltech Board considered to be the best route for the successful development and commercialisation of CDP870 given the terms proposed, the strength of UCB's specialist sales network and the relevant expertise of UCB's senior management. Celltech's management believe that, based on like for like commercial assumptions, the CDP870 agreement with UCB is of broadly equivalent value to the prior agreement with Pfizer. This agreement with UCB, which is not conditional on completion of the Offer, was announced on 18 May 2004.

        During the course of the CDP870 discussions, UCB also proposed combining the Celltech and UCB businesses in their entirety by way of an offer for Celltech by UCB. Having considered UCB's Offer, the Celltech Board has concluded that the Offer, which was announced on 18 May 2004 and full details of

6

which are set out in this document, should unanimously be recommended to shareholders. For further information regarding the background to the Offer, see paragraph 9(h) of Appendix V to this document.

7      Management and Employees

        UCB has assured the Celltech Board that, following the Offer becoming or being declared unconditional in all respects, the existing employment rights, including pension rights, of all employees of the Celltech Group will be fully safeguarded.

8      Celltech Share Plans

        The Offer will (subject to compliance with any applicable local laws) extend to any Celltech Shares issued fully paid (or credited as fully paid) or unconditionally allotted or issued while the Offer remains open for acceptance (or such earlier date as UCB may, subject to the City Code and the Exchange Act, determine, not being earlier than the date on which the Offer becomes unconditional as to acceptances), as a result of the exercise of options granted under the Celltech Share Plans. As soon as practicable after the Offer becomes or is declared unconditional in all respects, appropriate proposals will (taking into account any local laws) be made to the holders of options under the Celltech Share Plans.

9      Irrevocable undertakings

        UCB has received irrevocable undertakings to accept (or procure the acceptance of) the Offer from Celltech Directors and certain members of their immediate families in respect of, in aggregate, 493,029 Celltech Shares, representing all of the Celltech Shares beneficially owned and controlled by them and representing approximately 0.18 per cent. of the existing issued share capital of Celltech. Such undertakings cease to be binding only if the Offer lapses or is withdrawn.

10    Inducement Fee

        Celltech and UCB have entered into an agreement under which Celltech has agreed to pay UCB a sum of £15.25 million in certain circumstances. Further details of the inducement fee arrangement are set out in paragraph 15 in the letter from Lazard set out in Part II of this document and in paragraph 9(a) of Appendix V of this document.

11    Taxation

        Your attention is drawn to paragraph 16 of the letter from Lazard set out in Part II of this document. If you are in any doubt as to your tax position or are subject to taxation in any jurisdiction other than the United Kingdom or the United States, you should consult an appropriate professional adviser immediately.

12    Action to be taken to accept the Offer

        Your attention is drawn to the letter from Lazard in Part II of this document, the Appendices to this document and the accompanying Form of Acceptance or (if you are a holder of Celltech ADSs) Letter of Transmittal. The procedure for acceptance of the Offer in relation to Celltech Shares is set out in paragraph 18 of the letter from Lazard in Part II of this document and in the Form of Acceptance or (if you are a holder of Celltech ADSs) the Letter of Transmittal.

        A separate step-by-step guide on how to fill in your Form of Acceptance has been enclosed with this document.

        If you are a holder of Celltech Shares, in order to accept the Offer, you should complete and return the accompanying Form of Acceptance, whether or not your Celltech Shares are in CREST, in accordance with the instructions thereon as soon as possible and, in any event, so as to be received by post or by hand (during normal business hours only) by Capita IRG Plc, Corporate Actions, PO Box 166, The Registry, 34 Beckenham Road, Beckenham, Kent BR3 4TH, by no later than 3.00 p.m. (London time) on 17 June 2004.

        If you require further assistance on how to complete the Form of Acceptance, please call the Helpline. Please note that the Helpline will be unable to advise you on whether or not to accept the Offer or whether you should elect for the Loan Note Alternative.

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        Your decision as to whether to elect to receive cash or Loan Notes will depend on your individual circumstances, including your tax position. Paragraph 16 of the letter from Lazard set out in Part II of this document sets out certain implications of acceptance of the Offer in relation to United Kingdom and United States taxation. If you are in any doubt about the actions you should take, you should immediately consult your stockbroker, bank manager, solicitor, accountant or other independent financial adviser, duly authorised under the Financial Services and Markets Act 2000, if you are resident in the United Kingdom or, if not, another appropriately authorised independent financial adviser.

        If you are a holder of Celltech ADSs, in order to accept the Offer you should complete the Letter of Transmittal in accordance with paragraph 18(c) of the letter from Lazard and paragraph 2 of Part C of Appendix I to this document. If you require further assistance on how to complete the Letter of Transmittal, please call the Helpline.

13    Recommendation

        The Celltech Board, which has been so advised by Morgan Stanley and JPMorgan, considers the terms of the Offer to be fair and reasonable. In providing advice to the Celltech Board, Morgan Stanley and JPMorgan have taken into account the commercial assessments of the Celltech Board.

        Accordingly, the Celltech Board unanimously recommends that Celltech Shareholders accept the Offer, as Celltech Directors and certain members of their immediate families have irrevocably undertaken to do in respect of all of the Celltech Shares beneficially owned and controlled by them representing, in aggregate, 493,029 Celltech Shares and representing approximately 0.18 per cent. of the issued share capital of Celltech.

Yours faithfully

Dr Peter Fellner
Chairman

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PART II

LETTER FROM LAZARD

LAZARD & CO., LIMITED 50 Stratton Street, London W1J 8LL Authorised and regulated by the Financial Services Authority Member of the London Stock Exchange Registered in England no. 162175

        19 May 2004

To:    Celltech Shareholders and to the holders of Celltech ADSs and, for information only, to participants in the Celltech Share Plans

Dear Shareholder or ADS holder,

RECOMMENDED CASH OFFER BY UCB FOR CELLTECH

1      Introduction

        On 18 May 2004, the boards of UCB and Celltech announced that they had reached agreement on the terms of a recommended cash offer for the entire issued and to be issued ordinary share capital of Celltech. The Offer is being made (outside the United States) by Lazard on behalf of UCB and (in the United States) by UCB itself. The offer and this document are subject to the applicable requirements of both the City Code and the Exchange Act, subject to customary exemptions granted by the SEC in relation to the Offer.

        Your attention is drawn to the letter of recommendation from the Chairman of Celltech in Part I of this document, which sets out the reasons why the Celltech Board, which has been so advised by Morgan Stanley and JPMorgan, considers the terms of the Offer to be fair and reasonable and unanimously recommends that all Celltech Shareholders accept the Offer, as Celltech Directors and certain members of their immediate families have irrevocably undertaken to do (or procure to be done) in respect of all of the Celltech Shares beneficially owned and controlled by them amounting to, in aggregate, 493,029 Celltech Shares, representing approximately 0.18 per cent. of the existing issued share capital of Celltech.

        In providing advice to the Celltech Board, Morgan Stanley and JPMorgan have taken into account the commercial assessments of the Celltech Board.

2      Summary of the Offer

        Lazard, on behalf of UCB (outside the United States), and UCB itself (inside the United States) offer to acquire (on the terms and subject to the conditions set out in this document and in the Acceptance Forms) the entire issued and to be issued share capital of Celltech for cash. The Offer is being made on the following basis:

        The Offer values the entire issued and to be issued share capital of Celltech at approximately £1,530 million.

        The Offer represents a premium of:

•

27.8 per cent. to the Closing Price of 430.5 pence per Celltech Share on 17 May 2004, the last business day prior to the announcement by Celltech of the Offer;

•

26.3 per cent. to the average Closing Price of 435.6 pence per Celltech Share during the three months prior to 17 May 2004; and

•

44.7 per cent. to the average Closing Price of 380.1 pence per Celltech Share during the twelve months prior to 17 May 2004.

        Celltech Shares (including those represented by Celltech ADSs) will be acquired pursuant to the Offer fully paid and free from all liens, charges, equitable interests, encumbrances, rights of pre-emption

9

and other third party rights or interests of any nature whatsoever and together with all rights attaching now or hereafter attaching thereto including, without limitation, voting rights and the right to receive and retain all dividends and other distributions (if any), declared, made or paid on or after the date of the Announcement.

        The Offer (including the Loan Note Alternative) will be subject to the conditions and further terms set out or referred to in Appendix I to this document and in the Acceptance Forms.

        If you are a Celltech Shareholder, to accept the Offer you should return the Form of Acceptance whether or not your Celltech Shares are in CREST, together with all other required documents (such as your share certificate(s)), as soon as possible and, in any event, so as to be received by Capita IRG Plc, Corporate Actions, PO Box 166, The Registry, 34 Beckenham Road, Beckenham, Kent BR3 4TH by no later than 3.00 p.m. (London time) on 17 June 2004.

        The procedure for acceptance of the Offer if you are a Celltech Shareholder is set out in paragraph 18 below and in paragraph 1 of Part C of Appendix I to this document, and in the accompanying Form of Acceptance.

        In addition, a separate step-by-step guide to completing the Form of Acceptance has been sent to you with this document.

        If you are a holder of Celltech ADSs, you should follow the instructions set out in paragraph 18(c) below and in paragraph 2 of Part C of Appendix I to this document.

3      Loan Note Alternative

        Celltech Shareholders (other than certain Overseas Shareholders) who validly accept the Offer will be able to elect to receive Loan Notes instead of some or all of the cash to which they would otherwise become entitled under the terms of the Offer. The Loan Note Alternative is being made available on the following basis:

for every whole £1 in cash consideration £1 nominal value of Loan Notes

        The Loan Notes, which will be governed by English law, will be unsecured and will be issued credited as fully paid in amounts and integral multiples of £1 nominal value. All fractional entitlements to the Loan Notes will be disregarded. No application will be made for the Loan Notes to be listed or dealt on any stock exchange.

        The Loan Notes will bear interest at 0.75 per cent. below six month sterling LIBOR (as described in paragraph 2 of Appendix II). Interest will be payable by six-monthly instalments in arrears (less any tax required to be withheld) on 31 March and 30 September in each year. The first payment of interest will be made on 31 March 2005 (the "First Payment Date"). On the First Payment Date, interest will be paid in respect of the period from (and including) the first date of issue of any of the Loan Notes to (but excluding) the First Payment Date. The Loan Notes will be redeemable in whole or in part for cash at the option of Noteholders on the First Payment Date and on subsequent interest payment dates. No Loan Notes may be redeemed before the First Payment Date. In certain circumstances (set out in the Loan Note Instrument), UCB will have the right to redeem all of the Loan Notes. If not previously redeemed, all outstanding Loan Notes will be redeemed on 30 September 2010.

        No Loan Notes will be issued unless, on or before the date on which the Offer becomes or is declared unconditional in all respects, valid elections have been received in respect of at least £5 million in nominal value of Loan Notes. If insufficient elections are received, Celltech Shareholders electing for the Loan Note Alternative will instead receive cash in accordance with the terms of the Offer.

        Subject as aforesaid, the Loan Note Alternative will remain open for acceptance for so long as the Offer remains open for acceptance. The Loan Note Alternative will be conditional upon the Offer becoming or being declared unconditional in all respects.

        Lazard, financial adviser to UCB, has advised that based on market conditions on 18 May 2004 (the last practicable date prior to the posting of this document), in its opinion, if the Loan Notes had been in issue on that date, the value of each £1 nominal of Loan Notes would have been approximately 99 pence.

        Celltech Shareholders and holders of Celltech ADSs who are not resident in the United Kingdom should refer to paragraph 17 below and paragraph 6 of Part B of Appendix I.

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4      Irrevocable undertakings

        UCB has received irrevocable undertakings to accept (or procure the acceptance of) the Offer from Celltech Directors and certain members of their immediate families in respect of all of the Celltech Shares beneficially owned and controlled by them amounting to, in aggregate, 493,029 Celltech Shares, representing approximately 0.18 per cent. of the existing issued share capital of Celltech.

5      Background to and reasons for the Offer

        The combination of UCB and Celltech will create a European-based company which will be one of the largest biopharmaceutical companies in the world. In a number of important areas the combined group will benefit from a high degree of complementarity:

•

complementary product offerings: the combined group will have strong positions in specialty therapeutic areas such as Central Nervous System (Epilepsy, Multiple Sclerosis, Parkinson's Disease, etc.), Inflammation (Respiratory, Rheumatology, Gastro-Enterology) and Oncology;

•

strengthened research and development: the combined group will benefit from Celltech's innovative expertise in biotechnological R&D, especially monoclonal antibodies, and UCB's expertise in pharmaceutical chemistry. This will result in a significantly strengthened combination of small and large molecule discovery and development expertise allowing the combined group to increase the flow of drug development candidates;

•

stronger and broader commercial operations (US, Europe and Asia): the combined group will be better positioned, through its enhanced global presence, to commercialise and launch new products, particularly in the specialist areas in which the combination will focus and where UCB has already demonstrated its ability to launch and propel products successfully to market leadership (e.g. Keppra); and

•

the blending of skills and culture, through the successful combination of competencies at all levels of the combined entity, will accelerate innovation and profitable growth.

        The R&D operations of the combined group will have its headquarters in Slough, UK.

        Dr Göran Ando will be deputy chief executive officer of the combined group, Mr Peter Allen will be in charge of integration and Dr Melanie Lee will be head of the combined group's R&D operations.

        On 18 May 2004, UCB and Celltech entered into a co-exclusive world-wide collaboration agreement for the research, development and commercialisation of CDP870, Celltech's anti-TNF-alpha PEGylated antibody fragment, for all indications outside of Crohn's disease in North America and major European markets. This collaboration agreement is not conditional upon the success of the Offer for Celltech by UCB.

6      Financial Effects of the Transaction

        The cash consideration due under the Offer will be financed from the facilities described in paragraph 7 of Appendix V. It is expected that the transaction will be earnings accretive post synergies and before goodwill and other intangibles amortisation, after the second full year.

7      Information on the Celltech Group and Celltech current trading

        Celltech is one of the largest European-based biopharmaceutical companies, possessing significant discovery and development capabilities, a broad product pipeline, and an international pharmaceutical business, with operations in the United States and Europe. It derives revenues from the licensing of its technologies and products and the sale of pharmaceutical products through its pharmaceutical business.

        The discovery and development activities are focused on treatments for auto-immune and inflammatory disorders and oncology. Its pipeline includes candidates comprising new chemical entities and antibody-based therapeutics, in pre-clinical or clinical development and marketing licence registration. Its technology base includes a leading position in antibody engineering and extensive medicinal chemistry capabilities. Celltech has a range of discovery, development and commercialisation collaborations with leading pharmaceutical and biotechnology companies including: Abgenix, Amgen, AstraZeneca, Biogen Idec, Johnson & Johnson, Merck, NeoGenesis, Seattle Genetics and Wyeth.

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        Celltech Shares are traded on the London Stock Exchange and Celltech ADSs are listed on the New York Stock Exchange. Cazenove & Co. Limited acts as broker to Celltech.

        Celltech announced its preliminary results for the year ended 31 December 2003 on 16 March 2004 and posted the annual report and accounts for such period to Celltech Shareholders during the week commencing 19 April 2004. For the year ended 31 December 2003, in accordance with generally accepted accounting principles in the United Kingdom, Celltech reported turnover of £353.3 million (2002: £329.6 million) with operating profit pre exceptional items and goodwill of £49.5 million (2002: £49.0 million) and net assets of £505.9 million (2002: £564.4 million). The Celltech Board considers that Celltech's business will continue to perform in line with the Celltech Board's expectations.

        Financial information on the Celltech Group is contained in Appendix III to this document.

8      Information on the UCB Group and UCB current trading

        UCB is a world-class pharmaceutical and specialty chemical company. UCB is headquartered in Brussels (Belgium) and employs about 11,500 people, of whom more than 6,600 are in the pharmaceutical sector.

        UCB operates in two industrial sectors: Pharma and Surface Specialties. It currently comprises the parent company, UCB S.A., together with about 120 subsidiaries and associated companies in Europe, the Americas and Asia. UCB also operates on a worldwide basis through its agents, distributors and licensees.

        The Pharma Sector researches, produces and markets prescription medical products, particularly in the fields of Allergy/Asthma and Neurology. Over three quarters of the UCB Group's expenditure on research and development is in the Pharma Sector, where it accounts for about 15 per cent. of turnover. UCB is also present in biotechnology, through UCB-Bioproducts, making available peptides by extraction or synthesis to the scientific community.

        Surface Specialties is focused on the manufacture of technically innovative products and solutions for surface applications. There are two business units: Coating Resins & Additives and Films & Adhesives.

        UCB's shares are traded on Euronext. Based on the Closing Price of Euro 35.10 per UCB Share on 17 May 2004 (the last business day prior to the date of the Announcement), UCB has a market capitalisation of approximately Euro 5.1 billion.

        For the year ended 31 December 2003, in accordance with generally accepted accounting principles in Belgium, UCB reported turnover of Euro 2,966 million (2002: Euro 2,514 million) with group profit before interest, tax and exceptional items of Euro 487 million (2002: Euro 503 million) and net assets of Euro 1,784 million (2002: Euro 1,565 million).

        Further to UCB's press release of 4 February 2004, UCB still remains very confident about the favourable prospects for its results (at constant exchange rates), which will be influenced, on one hand by the growth in pharmaceutical specialties (Neurology and Allergy) and, on the other hand, by an increased contribution from synergies generated in the Surface Specialties sector.

        Financial information on UCB is contained in Appendix IV to this document.

9      Management and employees

        The Board of UCB has confirmed that, following the Offer being declared unconditional in all respects, the existing employment rights, including pension rights, of all employees of the Celltech Group will be fully safeguarded.

10    The Celltech Directors and the effect of the Offer on their interests

        Details of the interests of the Celltech Directors in Celltech Shares are set out in paragraph 4 of Appendix V of this document. The effect of the Offer on such interests does not differ from its effect on the interests of any other holder of Celltech Shares or participant in the Celltech Share Plans.

11    Disclosure of Interests in Celltech

        Save for the 493,029 Celltech Shares in respect of which UCB has received irrevocable undertakings to accept the Offer, neither UCB nor any of the directors of UCB nor any other UCB subsidiary, nor, so far as UCB is aware, any person acting in concert with UCB for the purposes of the Offer, owns, controls

12

or holds any Celltech Shares or any securities convertible or exchangeable into, or rights to subscribe for, purchase or holds any options to purchase any Celltech Shares or has entered into any derivative referenced to Celltech Shares which remains outstanding.

12    Celltech Share Plans

        Appropriate proposals in relation to the arrangements for participants in the Celltech Share Plans will be sent to optionholders as soon as practicable after the Offer has become or is declared unconditional in all respects.

        All existing options under the Celltech Share Plans, which are not already exercisable, will generally become exercisable in accordance with the rules of the Celltech Share Plans for a period ranging from one to six months (depending on the rules of the particular plan) following the Offer becoming, or being declared, unconditional in all respects. This period may be reduced if UCB commences the compulsory acquisition procedure.

        If an optionholder does not accept the Offer in respect of Celltech Shares acquired under any Celltech Share Plan, it is UCB's intention to acquire compulsorily such shares as described in paragraph 14 below. Any options which are not exercised will lapse no later than six months following the Offer becoming, or being declared, wholly unconditional (depending on the rules of the particular plan).

13    Collaboration Agreement

        On 18 May 2004, Celltech announced that it had entered into an agreement with UCB for the world-wide development and marketing of CDP870, Celltech's anti-TNF-alpha PEGylated antibody fragment.

        Under the terms of this agreement, Celltech granted UCB co-exclusive worldwide rights to develop and commercialise CDP870. The licence is exclusive for rheumatoid arthritis and other indications, excluding Crohn's disease. UCB will be responsible for the conduct of future clinical studies and all commercialisation activities with CDP870 other than in Crohn's disease, and will pay Celltech a significant royalty on sales in these indications. UCB will also make progress-related payments to Celltech dependent upon attaining certain project related milestones. Celltech has retained manufacturing rights and will supply all CDP870 material for commercialisation, and will discharge all royalties due to third parties. Celltech has retained exclusive rights for the development and commercialisation of CDP870 in Crohn's disease in North America, major European markets, Australia and New Zealand, with UCB having development and commercialisation rights in other territories.

        This agreement is not conditional upon the success of the Offer for Celltech by UCB.

14    Compulsory acquisition, delisting and re-registration

        If UCB receives acceptances under the Offer in respect of, and/or otherwise acquires, 90 per cent. or more of the Celltech Shares to which the Offer relates and the Offer becomes or is declared unconditional in all respects, UCB intends to exercise its rights pursuant to the provisions of sections 428 to 430F (inclusive) of the Companies Act to acquire compulsorily the remaining Celltech Shares to which the Offer relates.

        After the Offer becomes or is declared unconditional in all respects, UCB intends to procure the making of an application by Celltech to the UKLA for the cancellation of the listing of the Celltech Shares on the Official List and to the London Stock Exchange for the cancellation of the admission to trading of the Celltech Shares on its market for listed securities. It is anticipated that cancellation of listing and trading will take effect no earlier than 20 Business Days after the Offer becomes or is declared unconditional in all respects. UCB also intends to procure that Celltech applies for delisting of the Celltech ADSs from the New York Stock Exchange. Such delistings would significantly reduce the liquidity and marketability of any Celltech Securities not tendered into the Offer. UCB may also request that Celltech terminate the existing deposit agreement through which the ADS programme is operated.

        It is also proposed that, following the Offer becoming or being declared unconditional in all respects and after the Celltech Shares are delisted, Celltech will be re-registered as a private company under the relevant provisions of the Companies Act.

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15    Inducement fee

        Celltech and UCB have entered into an agreement under which Celltech has agreed to pay to UCB a sum of £15.25 million (being approximately one per cent. of the value of the Offer) in the event that (a) the Celltech Directors withdraw their unanimous recommendation of the Offer or recommend an alternative transaction, resulting in any person other than UCB (or any person acting in concert with UCB as defined in the City Code) acquiring control of Celltech (as defined in the City Code) or a substantial part of the business of Celltech; or (b) before the Offer lapses or is withdrawn without becoming or being declared wholly unconditional, any person (other than UCB or a person acting in concert (as defined in the City Code) with UCB) announces an intention to make a competing offer however effected, to acquire the entire issued share capital of Celltech (other than Celltech Shares owned by such third party or persons acting in concert with it) and, at any time, the competing offer becomes or is declared wholly unconditional.

16    Taxation

(A)  UK Taxation

        The following paragraphs, which are intended as a general guide only and are based on current UK legislation and Inland Revenue practice, summarise certain limited aspects of the UK taxation consequences of acceptance of the Offer. They relate only to the position of Celltech Shareholders who (unless the position of non-UK resident Celltech Shareholders is expressly referred to) are resident in or individuals ordinarily resident in the United Kingdom for tax purposes and who hold their Celltech Shares beneficially as an investment.

(i)    Taxation of Chargeable Gains

        Liability to United Kingdom taxation of chargeable gains in respect of the disposal of Celltech Shares pursuant to the Offer will depend on each Celltech Shareholder's individual circumstances and on whether a Celltech Shareholder accepts the consideration in cash or elects for the Loan Note Alternative.

(a)   Cash

To the extent a Celltech Shareholder receives cash under the Offer, this will constitute a disposal or part disposal of his Celltech Shares for the purposes of United Kingdom taxation of chargeable gains. Such a disposal may give rise to a liability for United Kingdom taxation of chargeable gains depending on the Celltech Shareholder's individual circumstances (including the availability of exemptions or allowable losses).

An alternative treatment may be possible where a Celltech Shareholder opts for a mixture of cash and Loan Notes. If the amount of cash received by the Celltech Shareholder is "small", as compared with the value of his Celltech Shares, the receipt of the cash will not, unless the Celltech Shareholder elects otherwise, trigger a disposal at that time. A disposal will be triggered only when his Loan Notes are disposed of and the amount of the cash received will be deducted from his chargeable gains acquisition cost in the Loan Notes. Current Inland Revenue practice is to regard a sum as "small" for these purposes if either (i) it is five per cent. or less of the value of the Celltech Shares held by the particular Celltech Shareholder; or (ii) it is £3,000 or less, regardless of whether it satisfies the five per cent. test. The advisability of adopting this alternative treatment will depend upon a Celltech Shareholder's individual circumstances, in particular the availability to a Celltech Shareholder of any reliefs or exemptions from UK taxation on chargeable gains in the tax year in which the cash is received.

(b)   Loan Notes

To the extent a Celltech Shareholder who (either alone or together with persons connected with him) does not hold more than five per cent. of, or of any class of, the shares in, or debentures of Celltech, receives Loan Notes under the Offer, he should be treated as not having made a disposal of his Celltech Shares for the purposes of UK taxation of chargeable gains.

For an individual or other Celltech Shareholder who is not within the charge to UK corporation tax, the Loan Notes should not constitute qualifying corporate bonds for the purposes of UK taxation of chargeable gains. Accordingly, for such a Celltech Shareholder any gain or loss which would otherwise

14

have arisen on a disposal of his Celltech Shares should be "rolled-over" into the Loan Notes so that the Loan Notes will be treated as the same asset as the Celltech Shares, acquired at the same time as the Celltech Shares and for the same acquisition cost. A disposal (including redemption or repayment) of Loan Notes by such a holder may give rise to a liability to UK taxation of chargeable gains. Any chargeable gain or allowable loss on disposal of the Loan Notes should be calculated taking into account the allowable original cost to the holder of acquiring the relevant Celltech Shares. Indexation allowance on that cost should be available (when calculating a chargeable gain but not an allowable loss) in respect of any period of ownership of the Celltech Shares up to April 1998. Thereafter some taper relief may be available which will reduce the amount of the chargeable gain realised on the disposal.

For a Celltech Shareholder within the charge to corporation tax, the Loan Notes will be qualifying corporate bonds for the purposes of UK taxation of chargeable gains. For such a holder, any gain or loss which would otherwise have arisen on a disposal of its Celltech Shares for a consideration equal to market value at the time of the exchange of the Celltech Shares for Loan Notes will be "held over" and deemed to arise on a subsequent disposal (including redemption or repayment) of the Loan Notes. No indexation allowance will be available for the period of ownership of the Loan Notes and, except to the extent any gain or loss which would have otherwise arisen on the disposal of its Celltech Shares was "held over" and crystallises on a subsequent disposal of the Loan Notes, no chargeable gain or allowable loss will arise on such a disposal. There may, however, be a charge to tax as income (see (b) below).

In certain circumstances, the above rules regarding the "roll-over" or "hold over" of any gain or loss will not apply to a Celltech Shareholder who (either alone or together with persons connected with him), holds more than five per cent. of, or of any class of, the shares in, or debentures of Celltech. Such persons are advised that an application for clearance has been made to the Inland Revenue under Section 138 of the Taxation of Chargeable Gains Act 1992 in respect of the Offer and, provided that such clearance is given, the benefit of the above rules regarding the "roll-over" or "hold over" of any gain or loss, will be available to such Celltech Shareholders. It is not a condition of the Offer that such clearance is obtained.

(ii)   Taxation of Income

        UK resident holders of Loan Notes will generally be liable to UK income tax or corporation tax on the amount of any interest received on their Loan Notes.

        Payments of interest on the Loan Notes may be made without withholding on account of UK income tax but will be made subject to withholding on account of Belgian income tax (currently 15 per cent.), except where holders certify to UCB that they were the legal owner or usufructuary (i.e. the person entitled to the right of enjoyment) of their Loan Notes during the entire interest period, are not resident in Belgium and do not hold the Loan Notes for the purposes of a trade or profession which they carry on in Belgium, and provided that certain other conditions are satisfied (see (C) Belgian Withholding Tax below). Holders of Loan Notes will receive a tax certificate with their Loan Notes to enable them to make this certification. In order to continue to receive payments of interest without withholding on account of Belgian income tax, holders of Loan Notes will be required to make such a certification for each interest payment for so long as they receive interest payments under the Loan Notes.

        UK resident holders of Loan Notes who are unable to, or who do not, provide the form of certificate referred to above will generally receive payments of interest subject to withholding on account of Belgian income tax.

        Where interest is paid subject to withholding on account of Belgian income tax, holders of Loan Notes who were unable to provide the form of certificate referred to above, may be able to claim a credit against their UK tax liability (if any) in respect of the relevant interest payment.

        In the case of individual or other non-corporate holders of Loan Notes, a charge to tax on income under the "accrued income scheme" may arise on a transfer of Loan Notes in respect of which interest has accrued since the preceding interest payment date.

        For a holder of Loan Notes within the charge to UK corporation tax, profits, gains and losses and fluctuations in the value of the Loan Notes (whether attributable to currency fluctuations or otherwise) will be taxed or relieved as income, broadly in accordance with the holder's authorised accounting method.

15

(iii) Celltech Share Plans

        Different tax treatment may apply to Celltech Shareholders who acquire their Celltech Shares by exercising options under the Celltech Share Plans, including a possible charge to income tax when such an option is exercised.

(iv)  Stamp Duty

        No stamp duty or stamp duty reserve tax should be payable by Celltech Shareholders as a result of accepting the Offer or on the issue of the Loan Notes.

        Under current Inland Revenue practice, no stamp duty or stamp duty reserve tax will be payable on the transfer or sale of (or agreement to transfer) Loan Notes.

        The above summary is intended only as a general guide to the taxation position under UK tax legislation and does not constitute tax or legal advice. Any person who is in doubt as to his taxation position or who requires more detailed information should consult his own professional tax adviser.

(B)  US Taxation

        The following summary describes certain material US federal income tax consequences that may be relevant to US Holders of Celltech Securities that are considering the Offer.

(i)

This summary provides general information only and is directed solely at US Holders who hold their Celltech Securities as capital assets and whose functional currency is the US dollar. This summary does not discuss all the tax consequences that may be relevant to US Holders in light of their particular investment circumstances, such as investors subject to special tax rules, including: persons who hold Celltech Securities through partnerships or other pass-through entities, banks or other financial institutions, thrifts, real estate investment trusts, regulated investment companies, insurance companies, dealers or traders in securities or currencies and tax-exempt investors, US Holders who received their Celltech Securities in return for services rendered or in connection with their employment, persons that own (directly or indirectly) 10 per cent. or more of Celltech voting stock or persons that hold their Celltech Securities as part of a hedge, straddle or other integrated transaction. This summary does not include descriptions of any alternative minimum tax consequences or the tax laws of any state or local government or of any foreign government that may be applicable to US Holders. Moreover, this summary does not discuss special tax provisions which may apply to individuals who relinquished their US citizenship or residence. This summary is based on the Internal Revenue Code of 1986, the Treasury regulations promulgated thereunder and administrative and judicial interpretations thereof, all as of the date hereof, and all of which are subject to change, possibly with retroactive effect, or different interpretations. No ruling has been requested from the Internal Revenue Service in connection with the Offer and no assurance can be given that the treatment described herein will be accepted by the Internal Revenue Service or, if challenged, by a US court.

The US federal income tax rules are complex. The discussion set forth below is included for general information only and may not be applicable depending upon a holder's particular situation. US Holders should consult their tax advisers with respect to the current and possibly future federal, state, local and foreign tax consequences to them of accepting the Offer.

(ii)

US Holders who sell their Celltech Securities pursuant to the Offer will generally recognise a taxable gain or loss equal to the difference between the US dollar value of their amount realised and their tax basis in their Celltech Securities. Such gain or loss generally will be US-source capital gain or loss and will be long-term if such US Holders held their Celltech Securities for more than one year. US Holders who receive US dollars in exchange for their Celltech ADSs will have an amount realised equal to the payments they receive. Although the law regarding US Holders who receive foreign currency in exchange for securities in a tender offer is unclear, cash basis taxpayers, or accrual basis taxpayers that make an election, who receive pounds sterling in exchange for their Celltech Securities should have an amount realised equal to the dollar value of the pounds sterling they receive, calculated using the spot rate on the settlement date. Accrual basis taxpayers that do not make an election who receive pounds sterling in exchange for their Celltech Securities should have an amount realised equal to the dollar value of the pounds sterling received, calculated using the spot rate on the date such US Holders' tenders are irrevocably accepted. US Holders who receive pounds sterling may recognise foreign currency exchange gain or loss due to fluctuations in the exchange rate upon a

16

subsequent conversion into US dollars of the pounds sterling received. Any foreign currency exchange gain or loss will be ordinary income or loss and will be US-source gain or loss.

The tax consequences for a US Holder could differ adversely from those described above if Celltech was a passive foreign investment company (a "PFIC") at any time during which the US Holder held Celltech Securities. Celltech's possible status as a PFIC must be determined annually. UCB does not have sufficient information to determine with certainty whether Celltech is or has ever been a PFIC.

If Celltech was a PFIC in any year during which a US Holder held Celltech Securities, the US Holder would be required (i) to pay tax on any gain from the sale of Celltech Securities at ordinary income (rather than capital gains) rates and (ii) to pay a special US addition to tax on gains from the sale of the Securities. Celltech will have been treated as a PFIC in any taxable year in which, after taking into account the income and assets of the corporation and certain subsidiaries pursuant to applicable "lookthrough rules," either (i) at least 75 per cent. of its gross income was "passive income" or (ii) at least 50 per cent. of the average value of its assets was attributable to assets which produce passive income or are held for the production of passive income. US Holders of Celltech Securities should consult their tax advisers regarding the potential application of the PFIC regime.

(iii)

Non-corporate US Holders may be subject to US federal backup withholding tax (imposed at a rate of 28 per cent.) and information reporting on payments received under the Offer if they fail to furnish and certify their correct taxpayer identification number in the manner required or otherwise fail to establish a basis for exemption. To prevent backup withholding, US Holders should complete Internal Revenue Service Form W-9 or a valid substitute form. Amounts withheld from payments to US Holders generally will be allowed as a credit against the US Holders' US federal income tax liability, provided that the required information is furnished to the Internal Revenue Service in a timely manner.

(C)  Belgian Withholding Tax

        UCB will be able to pay interest to a holder of the Loan Notes free from Belgian withholding tax so long as:

(i)

UCB receives a certificate from the holder certifying that he is the legal owner or usufructuary of the Loan Notes producing the interest, that he is a non-resident of Belgium, and does not hold the Loan Notes for the purposes of a trade or profession which he carries on in Belgium;



(ii)

the beneficial owner of the interest has been the legal owner or usufructuary of the Loan Notes during the entire period to which the interest relates;



(iii)

the Loan Notes remain in registered form; and



(iv)

UCB is a financial enterprise from the date of issue of the Loan Notes.

        For this purpose, UCB will be treated as a financial enterprise if it meets the following conditions: (1) it is a Belgian resident company or a Belgian permanent establishment of a foreign company; (2) it holds, during the tax year preceding the year during which interest is paid or a right to interest arises, shares having the nature of fixed financial assets whose investment value represents, on average, at least 50 per cent. of the total of the balance sheet at the end of the accounting year corresponding to the tax year; and (3) its shares are listed on a recognised stock exchange or it is controlled by a listed company (which means that at least 50 per cent. of its shares are directly or indirectly held by i) a Belgian resident company or by ii) a foreign company subject to corporate tax similar to the Belgian corporate tax, which does not benefit from a tax regime which is not applicable to all domestic companies or which is significantly more advantageous than the Belgian corporate tax). If UCB does not comply with these requirements in a particular accounting year, then all subsequent interest payments will be made under deduction of withholding tax (even if the requirements are satisfied at a later date), unless another exemption is available.

        If no exemption is available, payments of interest on the Loan Notes will be subject to Belgian withholding tax, currently at the rate of 15 per cent.

17

(D)  EU Directive on the taxation of savings income

        On June 3, 2003, the Council of the European Union adopted a directive on the taxation of savings income (Directive 2003/48/EC) (the "Directive") under which each Member State of the European Union ("Member State") will generally be required to provide to the tax authorities of another Member State details of payments of interest or other similar income paid by a person within its jurisdiction to or for an individual beneficiary resident in that other Member State.

        By way of exception (and for a transitional period only, which will end after agreement on exchange of information is reached between the European Union and certain non-European Union States), Belgium, Luxembourg and Austria will instead be required to impose a withholding tax, at a rate of 15 per cent. during the first three years from the date of application of the Directive, of 20 per cent. for the subsequent three years and of 35 per cent. thereafter, on such payments unless the beneficiary authorises the person making the payment to report the payment or presents a certificate from the relevant tax authority establishing exemption therefrom.

        The Directive will, subject to certain conditions being satisfied, apply from January 1, 2005.

        If and when the Directive comes into force, payments of interest on the Loan Notes made by a paying agent located in Belgium, Luxembourg or Austria to an individual beneficially entitled to the interest who is resident in another Member State will be made under deduction of withholding tax, unless the person beneficially entitled to the interest agrees to provide certain information to the paying agent in a form to be determined under the laws of Belgium, Luxembourg and Austria respectively.

17    Overseas Shareholders

        The attention of holders of Celltech Securities who are citizens or residents of jurisdictions outside the United Kingdom or the United States or who are holding shares for such citizens or residents and any person (including, without limitation, any nominee, custodian or trustee) who may have an obligation to forward any document in connection with the Offer outside the United Kingdom or the United States is drawn to paragraph 6 of Part B and to paragraph 1 of Part C of Appendix I to this document and to the relevant provisions of the Form of Acceptance, which should be read before taking any action.

        The availability of the Offer to persons not resident in, or not solely subject to the jurisdiction of, the United Kingdom or the United States may be affected by the laws of their relevant jurisdiction. Persons who are subject to the laws of any jurisdiction other than the United Kingdom or the United States should inform themselves about, and observe, any applicable legal or regulatory requirements of their jurisdiction. If you remain in any doubt, you should consult your professional adviser in the relevant jurisdiction without delay.

        The Offer in the United States is made solely by UCB, and neither Lazard nor any of its affiliates is making the Offer in the United States.

        Unless otherwise determined by UCB and permitted by applicable law and regulation, the Offer (including the Loan Note Alternative) is not being made, directly or indirectly, in or into a Restricted Jurisdiction and, subject to certain exemptions, the Offer is not capable of acceptance from or within a Restricted Jurisdiction. Accordingly, neither this document nor the Forms of Acceptance are being or may be mailed or otherwise forwarded, distributed or sent into or from a Restricted Jurisdiction where doing so may render any purported acceptance of the Offer invalid. All holders of Celltech Shares and Celltech ADSs (including nominees, trustees or custodians) who may have a contractual or legal obligation, or may otherwise intend, to forward this document and/or the Acceptance Forms, should read the further details in this regard which are contained in paragraph 6(c) of Part B of Appendix I to this document before taking any action.

        The Loan Notes which may be issued pursuant to the Loan Note Alternative have not been, and will not be, listed on any stock exchange and have not been and will not be registered under the Securities Act or under any relevant laws of any state or other jurisdiction of the United States, nor have the clearances been, nor will they be, obtained from the securities commission or similar authority of any province or territory of Canada and no prospectus has been or will be filed, or registration made, under any securities law of any province or territory of Canada, nor has a prospectus in relation to the Loan Notes been, nor will one be, lodged with or registered by the Australian Securities and Investments Commission nor have any steps been taken, nor will any steps be taken, to enable the Loan Notes to be offered in compliance with applicable securities laws of Japan or Belgium. Accordingly, unless an exemption under relevant

18

securities laws is available, Loan Notes may not be offered, sold re-sold or delivered, directly or indirectly, in, into or from the United States or a Restricted Jurisdiction or any jurisdiction in which an offer of Loan Notes would constitute a violation of relevant laws or require registration of the Loan Notes, or to or for the account or benefit of any US person or resident of a Restricted Jurisdiction.

        Notwithstanding the foregoing, UCB retains the right to permit the Offer to be accepted and any sale of securities pursuant to the Offer to be completed if, in its sole discretion, it is satisfied that the transaction in question can be undertaken in compliance with applicable law and regulation.

        Lazard Frères & Co. LLC is acting as the dealer manager for the Offer in the United States.

18    Procedure for acceptance of the Offer

        This section should be read in conjunction with the Acceptance Forms and Parts B and C of Appendix 1 to this document. The instructions on the Acceptance Forms are deemed to be part of the terms of the Offer.

(a)   If you hold Celltech Shares in certificated form (i.e. if you hold a share certificate)

(i)

Completion of Form of Acceptance

You will find enclosed with this document a Form of Acceptance for use in relation to the Offer. You should note that, if you hold Celltech Shares in both certificated and uncertificated form, you should complete a separate Form of Acceptance for each holding. If you hold Celltech Shares in certificated form, but under different designations, you should complete a separate Form of Acceptance in respect of each designation. Further Forms of Acceptance can be obtained from the Receiving Agent at Capita IRG Plc, Corporate Actions, PO Box 166, The Registry, 34 Beckenham Road, Beckenham, Kent BR3 4TH, or by telephoning the Helpline on 0870 162 3118 (if calling from the United Kingdom) (800) 261 1054 (if calling from the United States) or +44 208 639 2157 (if calling from elsewhere).

Your completed Form(s) of Acceptance should be lodged with the Receiving Agent at Capita IRG Plc, Corporate Actions, PO Box 166, The Registry, 34 Beckenham Road, Beckenham, Kent BR3 4TH, together with the relevant share certificates, other document(s) of title, letters of indemnity and supporting documents (if any), as soon as possible, but in any event so as to arrive not later than 3.00 p.m. (London time) on 17 June 2004. A reply-paid envelope is enclosed for your convenience.

Any Form of Acceptance received in an envelope postmarked in a Restricted Jurisdiction or otherwise appearing to UCB or its agents to have been sent from a Restricted Jurisdiction may be rejected as an invalid acceptance of the Offer. For further information for Overseas Shareholders, see paragraph 17 of this letter and paragraph 6 of Part B of Appendix I to this document.

(ii)

To accept the Offer in respect of your Celltech Shares

To accept the Offer in respect of your Celltech Shares, you should complete Box 2 on page 3 of the Form of Acceptance, and if appropriate Boxes 6, 7 and 8. If you do not insert a number in Box 2, your acceptance will be deemed to be in respect of all the Celltech Shares held by you. You must sign Box 4 of the Form of Acceptance in accordance with the instructions printed thereon. All Celltech Shareholders who are individuals (as opposed to companies) should sign Box 4 of the Form of Acceptance in the presence of a witness, who should also sign Box 4 in accordance with the instructions.

If you have any questions as to how to complete the Form of Acceptance, please telephone the Helpline on 0870 162 3118 (if calling from the United Kingdom), (800) 261 1054 (if calling from the United States) or +44 208 639 2157 (if calling from elsewhere).

(iii)

To elect for the Loan Note Alternative

To elect for the Loan Note Alternative in respect of some or all of the Celltech Shares for which you are accepting the Offer, you should complete Box 3 in addition to taking the actions described in paragraphs (i) and (ii) above. The attention of those holders of Celltech Shares considering accepting the Loan Note Alternative is drawn to paragraph 3 above, paragraph 4 of Part B of Appendix I and to Appendix II to this document.

19

(iv)

Share certificates not readily available or lost

Your completed, signed and (if appropriate) witnessed Form of Acceptance should be accompanied by the relevant share certificate(s) and/or other document(s) of title. If for any reason, your share certificate(s) and/or other document(s) of title is/are not readily available, you should nevertheless complete, sign and return your completed Form of Acceptance as stated above. You should send with the Form of Acceptance any share certificate(s) and/or other document(s) of title which you may have available, accompanied by a letter stating that the remaining documents will follow or that you have lost one or more of your share certificates and/or other documents of title and such certificate(s) and/or other document(s) of title should be forwarded as soon as possible thereafter.

If you have lost your share certificate(s) and/or other document(s) of title, you should write to Celltech's Registrar at Lloyds TSB Registrars, The Causeway, Worthing, West Sussex BN99 6DA, for a letter of indemnity for lost share certificate(s) and/or other document(s) of title which, when completed in accordance with the instructions given, should be returned to the Receiving Agent at Capita IRG Plc, Corporate Actions, PO Box 166, The Registry, 34 Beckenham Road, Beckenham, Kent BR3 4TH.

(b)   If you hold Celltech Shares in uncertificated from (that is, in CREST)

        You are reminded that if you are a CREST sponsored member, you should contact your CREST sponsor before taking any action.

(i)

Completion of Form of Acceptance

You will find enclosed with this document a Form of Acceptance for use in relation to the Offer. You should note that, if you hold Celltech Shares in both certificated and uncertificated form, you should complete a separate Form of Acceptance for each holding. If you hold Celltech Shares in uncertificated form, but under different member account IDs, you should complete a separate Form of Acceptance in respect of each member account ID. Further Forms of Acceptance can be obtained from the Receiving Agent at Capita IRG Plc, Corporate Actions, PO Box 166, The Registry, 34 Beckenham Road, Beckenham, Kent BR3 4TH, or by telephoning the Helpline on 0870 162 3118 (if calling from the United Kingdom), (800) 261 1054 (if calling from the United States) or +44 208 639 2157 (if calling from elsewhere).

Your completed Form(s) of Acceptance should be lodged with the Receiving Agent at Capita IRG Plc, Corporate Actions, PO Box 166, The Registry, 34 Beckenham Road, Beckenham, Kent BR3 4TH, as soon as possible, but in any event so as to arrive not later than 3.00 p.m. (London time) on 17 June 2004. A reply-paid envelope is enclosed for your convenience.

The instructions printed on the Form of Acceptance shall be deemed to form part of the terms of the Offer.

Any Form of Acceptance received in an envelope postmarked in a Restricted Jurisdiction or otherwise appearing to UCB or its agents to have been sent from a Restricted Jurisdiction may be rejected as an invalid acceptance of the Offer. For further information for Overseas Shareholders, see paragraph 17 of this letter and paragraph 6 of Part B of Appendix I to this document.

(ii)

To accept the Offer in respect of your Celltech Shares

To accept the Offer in respect of your Celltech Shares you must complete Box 2 of the Form of Acceptance and, as your Celltech Shares are in CREST, Box 5 and, if appropriate, Boxes 6, 7 and 8. If you do not insert a number in Box 2, your acceptance will be deemed to be in respect of all the Celltech Shares held by you. In all cases you must sign and date Box 4 of the Form of Acceptance in accordance with the instructions printed thereon. All Celltech Shareholders who are individuals (as opposed to companies) should sign Box 4 of the Form of Acceptance in the presence of a witness, who should also sign Box 4 in accordance with the instructions.

If you have any questions as to how to complete the Form of Acceptance, please telephone the Helpline on 0870 162 3118 (if calling from the United Kingdom), (800) 261 1054 (if calling from the United States) or +44 208 639 2157 (if calling from elsewhere).

(iii)

To elect for the Loan Note Alternative

To elect for the Loan Note Alternative in respect of some or all of the Celltech Shares for which you are accepting the Offer, you should complete Box 3 in addition to taking the actions described in

20

paragraphs (i) and (ii) above. The attention of those holders of Celltech Shares considering accepting the Loan Note Alternative is drawn to paragraph 3 above, paragraph 4 of Part B of Appendix I and to Appendix II to this document.

(iv)

Additional procedures for Celltech Shares held in uncertificated from (that is, in CREST)

If your Celltech Shares are held in uncertificated form, you should insert in Box 5 of the enclosed Form of Acceptance the participant ID and member account ID under which such Celltech Shares are held by you in CREST and otherwise complete and return the Form of Acceptance as described in (i), (ii) and (iii) above. In addition, you should take (or procure to be taken) the action set out below to transfer the Celltech Shares in respect of which you wish to accept the Offer to an escrow balance ("TTE Instruction"), specifying the Receiving Agent (in its capacity as a CREST participant under the Receiving Agent participant ID referred to below) as the escrow agent, as soon as possible and in any event so that the transfer to escrow settles not later than 3.00 p.m. (London time) on 17 June 2004.

If you are a CREST sponsored member, you should refer to your CREST sponsor before taking any action. Your CREST sponsor will be able to confirm details of your participant ID and the member account ID under which your Celltech Shares are held. In addition, only your CREST sponsor will be able to send the TTE Instruction to CRESTCo in relation to your Celltech Shares.

You should send (or, if you are a CREST sponsored member, procure that your CREST sponsor sends) a TTE Instruction to CRESTCo which must be properly authenticated in accordance with CRESTCo's specifications and which must contain, in addition to the other information that is required for a TTE Instruction to settle in CREST, the following details:

(a)

the Corporate Action ISIN. This is GB0001822765;



(b)

the number of Celltech Shares to be transferred to an escrow balance;



(c)

your participant ID. This must be the same participant ID as the participant ID that is inserted in Box 5 of the Form of Acceptance;



(d)

your member account ID. This must be the same member account ID as the member account ID that is inserted in Box 5 of the Form of Acceptance;



(e)

the participant ID of the escrow agent, Capita IRG Plc, in its capacity as a CREST receiving agent. This is RA10;



(f)

the member account ID of the escrow agent, Capita IRG Plc. This is CELLTECH;



(g)

the Form of Acceptance reference number. This is the reference number that is shown next to Box 5 on page 3 of the Form of Acceptance. This reference number should be inserted in the first eight characters of the shared note field on the TTE Instruction. Such insertion will assist Capita IRG Plc in matching the TTE to your Form of Acceptance. You should keep a separate record of this Form of Acceptance reference number for future reference;



(h)

the intended settlement date. This should be as soon as possible and in any event not later than 3.00 p.m. (London time) on 17 June 2004;



(i)

the Corporate Action Number for the Offer which is allocated by CRESTCo and can be found by reviewing the relevant Corporate Action Details in CREST; and



(j)

input with Standard Delivery instruction of 80.

        After settlement of the TTE Instruction, you will not be able to access the Celltech Shares concerned in CREST for any transaction or charging purposes. If the Offer becomes or is declared unconditional in all respects, the escrow agent will transfer the Celltech Shares concerned to itself in accordance with paragraph 8 of Part B and paragraph 1(d) of Part C of Appendix I.

        You are recommended to refer to the CREST manual published by CRESTCo for further information on the CREST procedures outlined above. For ease of processing, you are requested, wherever possible, to ensure that a Form of Acceptance relates to only one transfer to escrow.

        If no Form of Acceptance reference number, or an incorrect Form of Acceptance reference number, is included on the TTE Instruction, UCB may treat any amount of Celltech Shares transferred to an escrow balance in favour of the escrow agent specified above from the participant ID and member account ID identified in the TTE Instruction as relating to any Form(s) of Acceptance which relate(s) to the same

21

member account ID and participant ID (up to the amount of Celltech Shares inserted or deemed to be inserted on the Form(s) of Acceptance concerned).

        You should note that CRESTCo does not make available special procedures in CREST for any particular corporate action. Normal system timings and limitations will therefore apply in connection with a TTE Instruction and its settlement. You should therefore ensure that all necessary action is taken by you (or by your CREST sponsor) to enable a TTE Instruction relating to Celltech Shares to settle prior to 3.00 p.m. (London time) on 17 June 2004. In this regard you are referred in particular to those paragraphs of the CREST Manual concerning practical limitations of the CREST system and timings.

        UCB will make an appropriate announcement if any of the details contained in this paragraph alter for any reason in any respect that is material for Celltech Shareholders.

        You are reminded that if you are a CREST sponsored member, you should contact your CREST sponsor before taking any action.

(c)   If you hold Celltech ADSs

        The attention of holders of Celltech ADSs is drawn to paragraph 2 of Part C of Appendix I to this document and to the relevant provisions of the Letter of Transmittal.

        For a holder of Celltech ADSs to validly accept the Offer, either:

(i)

a properly completed and duly executed Letter of Transmittal (or a manually signed facsimile thereof), together with any other required signature guarantees or, in the case of a book-entry transfer, an Agent's Message, and any other documents required by the Letter of Transmittal, must be received by the Tender Agent at the appropriate address as set out in the Letter of Transmittal and either the Celltech ADRs evidencing such Celltech ADSs must be received by the Tender Agent at one of such addresses or such Celltech ADRs must be delivered pursuant to the procedure for book-entry transfer set out in paragraph 2 of Part C of Appendix I to this document (and a Book-Entry Confirmation received by the Tender Agent in accordance with such procedures); or



(ii)

such holder must comply with the guaranteed delivery procedures set out in paragraph 2 of Part C of Appendix I to this document.

        Acceptances using the guaranteed delivery procedures will not be taken into account in determining whether the Acceptance Condition has been satisfied unless the Celltech ADRs evidencing the Celltech ADSs or book-entry transfer of the Celltech ADSs to which the guaranteed delivery procedures relate have been received by the Tender Agent before the end of the Initial Offer Period.

        If you hold Celltech ADSs and are in any doubt as to the procedure for acceptance, please telephone the Helpline on (800) 261 1054 (toll free in the United States).

(d)   If you are the holder of Celltech Securities in any form whatsoever

(i)

Validity of acceptances

Without prejudice to Part B of Appendix I to this document and subject to the City Code, UCB reserves the right to treat as valid in whole or in part any acceptance of the Offer which is not entirely in order or which is not accompanied by (as applicable) the relevant transfer to escrow or the relevant share certificate(s), Celltech ADRs and/or other document(s) of title or which is received in a form or at a place or places other than as set out in this document or on the relevant Acceptance Form. In that event, no payment of cash or delivery of Loan Notes under the Offer will be required to be made until after the acceptance is entirely in order to UCB's satisfaction and (as applicable) the relevant transfer to escrow has been settled or the relevant share certificate(s), Celltech ADRs and/or other document(s) of title or indemnities satisfactory to UCB have been received by the Receiving Agent or the Tender Agent, as appropriate.

(ii)

No acknowledgement

No acknowledgement of receipt of Acceptance Form(s), Celltech share certificates, Celltech ADRs or other documents of title or documentation in respect of the Offer will be given by, or on behalf of UCB.

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19    Rights of Withdrawal

        Except to the extent of the exemptive relief which has been granted by the SEC, the Offer is subject to the US tender offer rules applicable to securities registered under the Exchange Act, as well as to the City Code. This has necessitated a number of changes from the procedures which normally apply to offers for companies governed by the City Code, including those applicable to the rights of holders of Celltech Shares and Celltech ADSs to withdraw their acceptance of the Offer.

        Under the Offer, holders of Celltech Securities will be able to withdraw their acceptances at any time during the Initial Offer Period but will not have any withdrawal rights during the Subsequent Offer Period, except in certain limited circumstances (see paragraph 3 of Part B of Appendix I to this document). The Subsequent Offer Period must remain open for at least 14 days but may be extended beyond that time until a further specified date or until further notice.

        The Offer will be deemed not to have been validly accepted in respect of any Celltech Shares or Celltech ADSs acceptances in respect of which have been validly withdrawn. However, the Offer may be accepted again in respect of any withdrawn Celltech Shares or Celltech ADSs by following one of the procedures described in paragraph 18 of this letter at any time prior to expiry of lapse of the Offer.

        It should be noted that by virtue of the conflicting provisions of the City Code and the Exchange Act, the Panel has agreed that the Acceptance Condition can be structured so that the Offer cannot become or be declared unconditional as to acceptances until such time as all other Conditions to the Offer have been satisfied, fulfilled or, to the extent permitted, waived. The Acceptance Condition in paragraph (a) of Part A of Appendix I to this document reflects this.

        Further details of these rights of withdrawal and the procedure for effecting withdrawals are set out in paragraph 3 of Part B of Appendix I to this document.

20    Settlement

(a)   Date of Payment

        The settlement procedure with respect to the Offer will be consistent with UK practice, which differs from US domestic tender offer procedures in certain material respects, particularly with regard to the date of payment.

        Subject to either the satisfaction, fulfilment or, to the extent permitted, waiver of all of the Conditions, settlement of consideration to accepting Celltech Shareholders and accepting holders of Celltech ADSs or their designated agents will be effected in the manner set out below:

(i)

in the case of acceptances received, complete in all respects, by the end of the Initial Offer Period, within 14 calendar days of such date; or

(ii)

in the case of acceptances received complete in all respects after such date but while the Offer remains open for acceptance, within 14 calendar days of such receipt.

(b)   Celltech Shares held in uncertificated form (that is, in CREST)

        Where an acceptance relates to Celltech Shares held in uncertificated form: (i) the cash consideration to which an accepting Celltech Shareholder is entitled will be paid by means of CREST by UCB procuring the creation of an assured payment obligation in favour of the accepting Celltech Shareholder's payment bank in respect of the cash consideration due, in accordance with the CREST assured payment arrangements; and (ii) definitive certificates for any Loan Notes will be despatched by post (or by such other method as may be approved by the Panel). No Loan Note certificates will be despatched to addresses in the United States or Restricted Jurisdictions.

        UCB reserves the right to settle all or any part of the cash consideration referred to above, for all or any accepting Celltech Shareholder(s), in the manner referred to in paragraph (c) below, if, for any reason, it wishes to do so.

(c)   Celltech Shares and Celltech ADSs held in certificated form

        Where an acceptance relates to Celltech Shares or Celltech ADSs held in certificated form, cheques drawn on a UK clearing bank in pounds sterling or on a New York City bank in US dollars, as appropriate, for the cash due and, where applicable, definitive Loan Note certificates for the appropriate nominal

23

amount of any Loan Notes will be despatched by post (or by such other method as may be approved by the Panel). No Loan Note certificates will be despatched to addresses in the United States and no payments or Loan Note certificates will be despatched to addresses in a Restricted Jurisdiction.

(d)   Lapsing of the Offer

        During the Initial Offer Period, if by the Closing Date the Conditions are not satisfied, fulfilled or, to the extent permitted, waived, the Offer will lapse. If the Offer lapses then: (i) in respect of Celltech Shares held in certificated form and Celltech ADSs, the relevant share certificate(s), Celltech ADRs and/or other documents of title will be returned by post (or by such other method as may be approved by the Panel) within 14 calendar days of the Offer lapsing to the person or agent whose name and address (outside a Restricted Jurisdiction) is set out in Box 1 (or, if applicable, Box 7) of the Form of Acceptance or to the person or agent whose name and address (outside a Restricted Jurisdiction) is set out in the Letter of Transmittal (as applicable) or, if none is set out, to the name and address of the person who is the first named holder at his or her registered address; (ii) in respect of Celltech Shares held in uncertificated form (that is, in CREST), the Receiving Agent will, immediately after the lapsing of the Offer (or within such longer period as the Panel may permit, not exceeding 14 calendar days after the lapsing of the Offer), give transfer from escrow instructions to CRESTCo to transfer all relevant Celltech Shares held in escrow balances and in relation to which it is the escrow agent for the purposes of the Offer to the original available balances of the holders of Celltech Shares concerned; and (iii) in respect of Celltech ADSs delivered by book-entry transfer into the Tender Agent's account at a Book-Entry Transfer Facility, Celltech ADSs will be credited to an account maintained at the appropriate Book-Entry Transfer Facility.

(e)   General

        All documents and remittances sent by, to or from holders of Celltech Shares and Celltech ADSs or their appointed agents will be sent at their own risk.

(f)    Currency of cash consideration

        Holders of Celltech Shares, whether located in the United States or elsewhere, will receive the cash consideration in pounds sterling.

        Holders of Celltech ADSs are entitled under the terms of the Offer to receive the cash consideration in pounds sterling. The pounds sterling consideration available to holders of Celltech ADSs is the same, per Celltech Share, as that offered to Celltech Shareholders. To facilitate the settlement of the Offer, unless they elect to receive pounds sterling, holders of Celltech ADSs will receive consideration converted into US dollars at the exchange rate obtainable on the spot market in London on the date the cash consideration is made available by UCB to the Tender Agent for delivery in respect of the relevant Celltech ADSs. Under no circumstances will interest on the purchase price of Celltech Securities be paid by UCB because of any delay in paying for any Celltech Shares (including Celltech Shares represented by Celltech ADSs).

        A holder of Celltech ADSs may receive such amount on the basis set out above only in respect of the whole of his holding of Celltech ADSs in respect of which he accepts the Offer. Holders of Celltech ADSs may not elect to receive both pounds sterling and US dollars.

        If you are a holder of Celltech ADSs and you wish to elect to receive cash consideration in pounds sterling instead of US dollars under the offer, you should complete the Box entitled "Pounds Sterling Payment Election" on the Letter of Transmittal in addition to taking the actions described in paragraph 18(c) above.

        The actual amount of US dollars received will depend upon the exchange rate prevailing on the day on which funds are made available to the Tender Agent by UCB. Holders of Celltech ADSs should be aware that the US dollar/pound sterling exchange rate which is prevailing on the date on which an election is deemed to be made to receive US dollars and on the dates of despatch and receipt of payment may be different from that prevailing on the day on which funds are made available to the Tender Agent by UCB. In all cases, fluctuations in the US dollar/pounds sterling exchange rate are at the risk of accepting holders of Celltech ADSs. UCB and its advisers or agents shall not have any responsibility with respect to the actual amount of cash consideration payable other than in pounds sterling.

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21    Further information

        Your attention is drawn to the further information contained in the appendices which form part of this document, and to the accompanying Acceptance Forms, which should be read in conjunction with this document. The appendices and the Acceptance Forms contain material information which may not be summarised elsewhere.

22    Action to be taken

        If you wish to accept the Offer you are urged to complete, sign and return the Form of Acceptance or, if you are a holder of Celltech ADSs, the Letter of Transmittal, together with all the documents required to be returned with it, as soon as possible and, in any event, so as to be received by the Receiving Agent or the Tender Agent, as appropriate, by no later than 3.00 p.m. (London time), 10.00 a.m. (New York City time), on 17 June 2004.

Yours faithfully,
for and on behalf of
Lazard & Co., Limited

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APPENDIX I

Conditions to, and Further Terms of, the Offer

Part A—Conditions to the Offer

        The Offer is subject to the following conditions:

(a)

valid acceptances of the Offer being received (and not, where permitted, withdrawn) by 3.00 p.m. (London time), 10.00 a.m. (New York City time) on 17 June 2004 or such later time(s) and/or date(s) as UCB may, subject to the rules of the City Code and in accordance with the Exchange Act, decide in respect of not less than 90 per cent. in nominal value (or such lesser percentage as UCB may decide) of the Celltech Shares (including Celltech Shares represented by Celltech ADSs) to which the Offer relates, provided that this condition will not be satisfied unless UCB and/or any of its wholly-owned subsidiaries shall have acquired, or agreed to acquire, pursuant to the Offer or otherwise, Celltech Shares (including Celltech Shares represented by Celltech ADSs) carrying, in aggregate more than 50 per cent. of the voting rights normally exercisable at a general meeting of Celltech, including for this purpose (to the extent, if any, required by the Panel) any such voting rights attaching to any Celltech Shares (including Celltech Shares represented by Celltech ADSs) that are unconditionally allotted or issued before the Offer becomes or is declared unconditional as to acceptances whether pursuant to the exercise of any outstanding subscription or conversion rights or otherwise, and for the purposes of this condition:



(i)

Celltech Shares (including Celltech Shares represented by Celltech ADSs) which have been unconditionally allotted but not issued shall be deemed to carry the voting rights they will carry on issue;



(ii)

the expression "Celltech Shares (including Celltech Shares represented by Celltech ADSs) to which the Offer relates" shall be construed in accordance with sections 428 to 430F of the Companies Act; and



(iii)

valid acceptances also shall be treated as having been received in respect of any Celltech Shares (including Celltech Shares represented by Celltech ADSs) which UCB shall, pursuant to section 429(8) of the Companies Act, be treated as having acquired or contracted to acquire by virtue of acceptance of the Offer;

provided further that unless UCB otherwise determines, this condition (a) shall be capable of being satisfied only at a time when all of the other conditions (b) to (m) inclusive have been either satisfied, fulfilled or, to the extent permitted, waived;

(b)

subject to condition (e) below:



(i)

the UK Office of Fair Trading ("OFT") indicating in terms reasonably satisfactory to UCB that it does not believe that the proposed acquisition of Celltech by UCB (the "Transaction") creates a relevant merger situation within the meaning of section 23 of the Enterprise Act 2002 ("EA"); or



(ii)

the OFT indicating in terms reasonably satisfactory to UCB that it has decided not to refer the Transaction or any part of it to the Competition Commission ("CC"); or



(iii)

the period for considering any merger notice given to the OFT under section 96 of the EA by UCB having expired without any such reference being made, provided that sections 100(1)(a), (d) and (f) of the EA do not apply in relation to such merger;



(c)

subject to condition (e) below, UCB obtaining an approval, or being deemed to have obtained an approval, from the German competition authority, the Federal Cartel Office ("Bundeskartellamt"), in terms reasonably satisfactory to UCB, in relation to the Transaction;



(d)

subject to condition (e) below, if the Transaction requires notification to the Austrian Cartel Court under the Austrian Cartel Act ("Cartel Act"), then:



(i)

the Austrian Cartel Court issuing a clearance certificate pursuant to Section 42b(1) Cartel Act, in terms reasonably satisfactory to UCB; or



(ii)

the four week period under Section 42b(1) Cartel Act expiring without an in-depth investigation having been opened or requested; or

26

(iii)

the Austrian Cartel Court not prohibiting the Transaction within a period of five months from the receipt of the complete notification; or



(iv)

the Austrian Cartel Court issuing a final order pursuant to Section 42b(2) no 1 Cartel Act which sets forth that the Transaction does not constitute a concentration within the meaning of Section 41 Cartel Act; or



(v)

the Austrian Cartel Court issuing a final order pursuant to Section 42b(2) no 3 Cartel Act which sets forth, in terms reasonably satisfactory to UCB, that the concentration will not be prohibited;



(e)

if a request to the European Commission is made by the competent authorities of one or more Member States under Article 22 of Council Regulation (EC) No 139/2004 (the "Regulation") in relation to the Transaction or any part of it and is accepted by the European Commission:



(i)

the European Commission issuing a decision pursuant to Article 6(1)(b) of the Regulation in terms reasonably satisfactory to UCB declaring that the Transaction or, if applicable, the relevant part of the Transaction, is compatible with the common market; and



(ii)

to the extent that the OFT retains jurisdiction over any part of the Transaction, one of the conditions set out in conditions (b)(i) to (iii) above being satisfied in relation to each such part of the Transaction; and



(iii)

to the extent that the Bundeskartellamt retains jurisdiction over any part of the Transaction, the condition set out in condition (c) above being satisfied in relation to each such part of the Transaction; and



(iv)

to the extent that the Austrian Cartel Court retains jurisdiction over any part of the Transaction, one of the conditions set out in conditions (d)(i)–(v) above being satisfied in relation to each such part of the Transaction;



(f)

all required filings having been made under the United States Hart-Scott-Rodino Antitrust Improvements Act of 1976 (as amended) and the rules and regulations thereunder and all applicable waiting periods with respect thereto having expired or been terminated as appropriate, in each case in connection with the Transaction;



(g)

all necessary notifications and filings required by law or regulation having been made, all or any applicable waiting and other time periods (including any extensions thereof (including requests for additional information)) under any applicable legislation or regulation of any jurisdiction having expired, lapsed or terminated and any approvals or clearances required by law or regulation having been obtained to the reasonable satisfaction of UCB as appropriate, in each case, in respect of the Offer, its implementation and the acquisition of any shares in, or control of, Celltech (or any member of the Celltech Group) by UCB or any member of the UCB Group and all necessary statutory and regulatory obligations in connection with the Offer in any jurisdiction having been complied with;



(h)

other than investigations and waiting periods listed in condition (g), no central bank, government or governmental, quasi-governmental, supranational, statutory or regulatory body or association, institution or agency (including any trade agency) or any court or other body (including any professional or environmental body) or person in any jurisdiction (each a "Relevant Authority") having decided to take, instituted or threatened any action, proceeding, suit, investigation, enquiry or reference or enacted, made or proposed and there not continuing to be outstanding any statute, regulation, order or decision that would or might reasonably be expected to:



(i)

make the Offer, its implementation or the acquisition or the proposed acquisition of any Celltech Shares (including Celltech Shares represented by Celltech ADSs) in, or control of, Celltech by any member of the UCB Group void, unenforceable or illegal or directly or indirectly prohibit or restrict, delay or interfere with the implementation of, or impose material additional conditions or obligations with respect to, or otherwise challenge, the Offer, its implementation or the acquisition of any shares in, or control of, Celltech by any member of the UCB Group;



(ii)

result in a delay in the ability of UCB or any member of the UCB Group, or render UCB or any member of the UCB Group unable, to acquire all of the Celltech Shares (including Celltech Shares represented by Celltech ADSs) or require a divestiture by UCB or any

27

member of the UCB Group of any Celltech Shares (including Celltech Shares represented by Celltech ADSs);

(iii)

require, prevent or materially delay the divestiture (or alter the terms of any proposed divestiture) by any member of the Wider UCB Group or any member of the Wider Celltech Group of all or any part of their respective businesses, assets or properties or impose any limitation on their ability to conduct all or any part of their respective businesses and to own any of their respective assets or properties to an extent which is material in the context of the Wider Celltech Group taken as a whole or the UCB Group taken as a whole (as the case may be);



(iv)

impose any material limitation on, or result in any material delay in, the ability of any member of the UCB Group to acquire or hold Celltech Shares (including Celltech Shares represented by Celltech ADSs) or other securities (or the equivalent) in any member of the Celltech Group or to exercise effectively, directly or indirectly, all or any rights of ownership of Celltech Shares (including Celltech Shares represented by Celltech ADSs) or other securities (or the equivalent) in, or to exercise management control over, any member of the Celltech Group or on the ability of any member of the Celltech Group to hold or exercise effectively, directly or indirectly, all or any rights of ownership of shares or other securities (or the equivalent) in, or to exercise management control over, any other member of the Celltech Group;



(v)

result in any member of the Celltech Group ceasing to be able to carry on business under any name which it presently does so to an extent which is material in the context of the Wider Celltech Group taken as a whole or the UCB Group taken as a whole (as the case may be);



(vi)

require any member of the UCB Group or of the Celltech Group to acquire or offer to acquire any shares or other securities (or the equivalent) in any member of the Celltech Group or any member of the Wider UCB Group owned by any third party (other than in the implementation of the Offer) to an extent which is material in the context of the Wider Celltech Group taken as a whole or the UCB Group taken as a whole (as the case may be);



(vii)

impose any limitation on the ability of any member of the Wider UCB Group or the Celltech Group to integrate or co-ordinate its business, or any part of it, with the businesses or any part of the businesses of any other member of the Wider UCB Group and/or the Celltech Group to an extent which is material in the context of the Wider Celltech Group taken as a whole or the UCB Group taken as a whole (as the case may be); or



(viii)

otherwise adversely affect the business, assets, financial or trading position or profits or prospects of any member of the Wider UCB Group or of the Wider Celltech Group in a manner which is adverse to and material in the context of the UCB Group taken as a whole or the Wider Celltech Group taken as a whole (as the case may be),

and all applicable waiting and other time periods during which any such Relevant Authority could decide to take, institute, implement or threaten any such action, proceeding, suit, investigation, enquiry or reference having expired, lapsed or been terminated;

(i)

all notifications and filings have been made in connection with the Offer and all authorisations, orders, grants, recognitions, confirmations, licences, consents, clearances, certificates, permissions and approvals ("Authorisations") necessary or appropriate in any jurisdiction for, in respect of or resulting from the Offer, its implementation or the proposed acquisition by UCB or any member of the UCB Group of any shares in Celltech or control of Celltech (or any member of the Celltech Group) being obtained in terms and in a form reasonably satisfactory to UCB from appropriate Relevant Authorities or from any persons or bodies with whom any member of the Wider UCB Group or the Celltech Group has entered into contractual arrangements and such authorisations, together with all authorisations necessary or appropriate for any member of the Celltech Group to carry on its business where such business is material in the context of the Wider Celltech Group as a whole remaining in full force and effect and no intimation of any intention to revoke, suspend, restrict or modify or not to renew any such authorisations having been made;



(j)

save as fairly disclosed in the annual report and accounts of the Celltech Group for the year ended 31 December 2003, or by or on behalf of Celltech to UCB, or as publicly announced to a

28

Regulatory Information Service by or on behalf of Celltech, (in each such case) prior to 18 May 2004, there being no provision of any agreement, arrangement, lease, licence, permit or other instrument to which any member of the Wider Celltech Group is a party or by or to which any such member or any of its assets is or may be bound, entitled or subject which, as a result of the making of the Offer, its implementation or the acquisition or proposed acquisition by UCB of any shares in, or change in the control or management of, Celltech or otherwise, would or might reasonably be expected to result in, and no event having occurred which, under any provision of any agreement, arrangement, licence, permit or other instrument to which any member of the Wider Celltech Group is a party or by or to which any such member or any of its assets may be bound, entitled or subject, could reasonably be expected to result in, to an extent which is, in any case, material in the context of the Wider Celltech Group taken as a whole:

(i)

any monies borrowed by or any other indebtedness (actual or contingent) of any such member being or becoming repayable or being capable of being declared repayable immediately or earlier than the stated repayment date or the ability of such member to borrow monies or incur any indebtedness being withdrawn or inhibited or any such arrangement, agreement, licence or instrument relating to any monies borrowed or indebtedness being terminated or modified or any onerous obligations arising or any action being taken or arising thereunder;



(ii)

the creation or enforcement of any mortgage, charge or other security interest over the whole or any part of the business, property or assets of any such member or any such security interest (whenever arising or having arisen) becoming enforceable;



(iii)

any assets or interests of any such member being or falling to be disposed of or charged or any right arising under which any such asset or interest could be required to be disposed of or charged other than in the ordinary course of business;



(iv)

the interest or business of any such member in or with any other person, firm or company (or any agreements or arrangements relating to such interest or business) being terminated or adversely modified or affected;



(v)

any such member ceasing to be able to carry on business under any name under which it presently does so;



(vi)

the value of any such member or its financial or trading position or profits or prospects being prejudiced or adversely affected;



(vii)

any such agreement, arrangement, licence or other instrument being terminated or adversely modified or any onerous obligation arising or any adverse action being taken or arising thereunder; or



(viii)

the creation of any liabilities (actual or contingent) by any such member;



(k)

since 31 December 2003 (except as publicly announced to a Regulatory Information Service by or on behalf of Celltech before 18 May 2004 or as fairly disclosed in the annual report and accounts of the Celltech Group for the year ended 31 December 2003, or by or on behalf of Celltech to UCB prior to 18 May 2004), no member of the Celltech Group having:



(i)

save as between Celltech and its wholly-owned subsidiaries prior to 18 May 2004 or upon the exercise of rights to subscribe for Celltech Shares pursuant to options granted under the Celltech Share Option Schemes prior to such date, issued or agreed to issue or authorised the issue of additional shares of any class, or securities convertible into or exchangeable for, or rights, warrants or options to subscribe for or acquire, any such shares or convertible securities;



(ii)

recommended, declared, paid or made or proposed to recommend, declare, pay or make any bonus issue, dividend or other distribution, whether payable in cash or otherwise, other than a distribution by any wholly-owned subsidiary of Celltech to Celltech or any of its wholly owned subsidiaries;



(iii)

save for transactions between Celltech and its wholly-owned subsidiaries, implemented or authorised any merger or demerger or acquired or disposed of or, other than in the ordinary course of business, transferred, mortgaged or charged, or created any other security interest over, any material asset or any right, title or interest in any material asset;

29

(iv)

implemented or authorised any reconstruction, amalgamation or scheme of arrangement;



(v)

other than between Celltech and its wholly-owned subsidiaries or between such wholly-owned subsidiaries, purchased, redeemed or repaid any of its own shares or other securities or reduced or made or authorised any other change in its share capital;



(vi)

other than between Celltech and its wholly-owned subsidiaries or between such wholly-owned subsidiaries, made or authorised any change in its loan capital or issued or authorised the issue of any debentures or incurred or increased any indebtedness or contingent liability;



(vii)

entered into, varied or terminated, or authorised the entry into, variation or termination of, any contract, commitment, agreement, proposal or arrangement (whether in respect of capital expenditure or otherwise) which is outside the ordinary course of trading or which is of a long-term, onerous or unusual nature or magnitude or which involves or could involve an obligation of a nature or magnitude which is material in the context of the Wider Celltech Group (taken as a whole) or which is or is likely to be restrictive on the business of any member of the Wider Celltech Group or the Wider UCB Group to an extent which is material in the context of the Wider Celltech Group or the UCB Group (as appropriate) taken as a whole;



(viii)

been unable, or admitted in writing that it is unable, to pay its debts or having stopped or suspended (or threatened to stop or suspend) payment of its debts generally or ceased or threatened to cease carrying on all or a substantial part of its business;



(ix)

taken any corporate action or had any legal proceedings started or threatened against it for its winding-up (voluntary or otherwise), dissolution or reorganisation (or for any analogous proceedings or steps in any jurisdiction) or for the appointment of a receiver, administrator, administrative receiver, trustee or similar officer (or for the appointment of any analogous person in any jurisdiction) of all or any of its assets and revenues or appointed any analogous person in any jurisdiction;



(x)

waived, compromised or settled any claim which is material;



(xi)

entered into or varied the terms of any service agreement or arrangement with any director or senior executive of Celltech;



(xii)

made or consented to any significant change to the terms of the trust deeds constituting the pension schemes established for its directors and/or employees and/or their dependants or to the benefits which accrue, or to the pensions which are payable thereunder, or to the basis on which qualification for or accrual or entitlement to such benefits or pensions are calculated or determined, or to the basis upon which the liabilities (including pensions) of such pension schemes are funded or made, or agreed or consented to any change to the trustees; or



(xiii)

entered into any contract, commitment or arrangement or passed any resolution or made any offer (which remains open for acceptance) with respect to, or proposed or announced any intention to effect or propose, any of the transactions, matters or events referred to in sub-paragraphs (i) to (xii) of this condition (k);



(l)

since 31 December 2003 (except as publicly announced to a Regulatory Information Service by or on behalf of Celltech before 18 May 2004 or as fairly disclosed in the annual report and accounts of the Celltech Group for the year ended 31 December 2003, or by or on behalf of Celltech to UCB prior to 18 May 2004):



(i)

no adverse change having occurred in the business, assets, financial or trading position or profits or prospects of any member of the Wider Celltech Group to an extent which is material to the Wider Celltech Group taken as a whole;



(ii)

no litigation, arbitration proceedings, prosecution or other legal proceedings having been threatened, announced, instituted or remaining outstanding by, against or in respect of any member of the Wider Celltech Group or to which any member of the Wider Celltech Group is a party (whether as claimant or defendant or otherwise) and no investigation by any Relevant Authority or other investigative body against or in respect of any member of the Celltech Group having been threatened, announced, instituted or remaining outstanding by, against or in respect of any member of the Wider Celltech Group which, in any such case,

30

might be reasonably likely to adversely affect any member of the Wider Celltech Group to an extent which is material to the Wider Celltech Group (taken as a whole); or

(iii)

no contingent or other liability having arisen which would or might be reasonably likely to adversely affect any member of the Wider Celltech Group to an extent which is material to the Wider Celltech Group taken as a whole; and



(m)

except as fairly disclosed to UCB in the annual report and accounts of the Celltech Group for the year ended 31 December 2003, or by or on behalf of Celltech prior to 18 May 2004, UCB not having discovered that:



(i)

any financial, business or other information concerning the Wider Celltech Group disclosed at any time by or on behalf of any member of the Wider Celltech Group is misleading, contains a misrepresentation of fact or omits to state a fact necessary to make the information contained therein not misleading and which is material in the context of the Wider Celltech Group (taken as a whole);



(ii)

any member of the Wider Celltech Group is subject to any liability, contingent or otherwise, which should have been but is not disclosed in the annual report and accounts of Celltech for the year ended 31 December 2003 and which is material in the context of the Wider Celltech Group (taken as a whole);



(iii)

any past or present member of the Wider Celltech Group has failed to comply in any material respect with any applicable legislation or regulations of any jurisdiction or any notice or requirement of any Relevant Authority with regard to the storage, disposal, discharge, spillage, release, leak or emission of any waste or hazardous or harmful substance or any substance likely to impair the environment or harm human or animal health or otherwise relating to environmental matters or that there has been any such storage, presence, disposal, discharge, spillage, release, leak or emission (whether or not the same constituted non-compliance by any person with any such legislation or regulation, and whenever the same may have taken place), any of which non-compliance would be likely to give rise to any material liability (whether actual or contingent) or cost on the part of any member of the Wider Celltech Group and which is material in the context of the Wider Celltech Group (taken as a whole); or



(iv)

there is, or is reasonably likely to be, any material obligation or liability (whether actual or contingent) to make good, repair, re-instate or clean up any property now or previously owned, occupied, operated or made use of or controlled by any past or present member of the Celltech Group under any environmental legislation, regulation, notice, circular or order of any Relevant Authority in any jurisdiction in each case to an extent which is material in the context of the Wider Celltech Group (taken as a whole).

        If UCB is required by the Panel to make an offer for Celltech Shares under the provisions of Rule 9 of the City Code, then UCB may make such alterations to any of the above conditions as are necessary to comply with the provisions of that Rule.

        UCB reserves the right to waive all or any of conditions (b) to (m) above inclusive, in whole or in part. The Offer will lapse unless all the above conditions are fulfilled or (if capable of waiver) waived or, where appropriate, determined by UCB to have been or remain satisfied by 3.00 p.m. (London time), 10.00 a.m. (New York City time) on 17 June 2004 (or such later date as UCB may determine, in accordance with the City Code and the Exchange Act). UCB shall be under no obligation to waive or treat as fulfilled any of conditions (b) to (m) inclusive by a date earlier than the latest date for the fulfilment thereof notwithstanding that the other conditions of the Offer may at such earlier date have been waived or fulfilled and that there are at such earlier date no circumstances indicating that any of such conditions may not be capable of fulfilment.

        If before 3.00 p.m. (London time), 10.00 a.m. (New York City time) on 17 June 2004 or the date when the Offer becomes unconditional as to acceptances (whichever is the later):

(i)

the OFT has referred the Offer to the Competition Commission; or



(ii)

the European Commission has initiated proceedings in respect of the Offer under Article 6(1)(c) of the Regulation,

the Offer will lapse.

31

        For the purposes of this Appendix:

        "Wider Celltech Group" means Celltech and its subsidiary undertakings, associated undertakings and any other undertakings in which Celltech and such undertakings (aggregating their interests) have a substantial interest; and

        "Wider UCB Group" means UCB and its subsidiary undertakings, associated undertakings and any other undertakings in which UCB and such undertakings (aggregating their interests) have a substantial interest.

        For these purposes "subsidiary undertaking", "associated undertaking" and "undertaking" have the meanings given by the Companies Act (but for these purposes ignoring paragraph 20(1)(b) of Schedule 4A to the Companies Act) and "substantial interest" means a direct or indirect interest in 20 per cent. or more of the equity capital of an undertaking.

32

Part B—Further Terms of the Offer

        Except where the context requires otherwise, any reference in this document and in the Acceptance Forms to:

(a)

the "Offer" will include the Loan Note Alternative and any other election available under the Offer and any revision, variation, renewal or extension thereof;

(b)

the "Offer becoming unconditional" will include the Offer being or becoming or being declared unconditional;

(c)

the "Offer Document" will mean this document and any other document containing the Offer; and

(d)

"acting in concert with UCB" will mean any such person acting or deemed to be acting in concert with UCB for the purposes of the City Code and/or the Offer.

        The following Further Terms apply, unless the context requires otherwise, to the Offer:

1      Acceptance Period

(a)

The Offer will initially be open for acceptance until 3.00 p.m. (London time), 10.00 a.m. (New York City time) on 17 June 2004.

(b)

Although no revision is envisaged, if the Offer is revised it will remain open for acceptance for a period of at least 14 calendar days (or such other period as may be permitted by the Panel that is in accordance with the Exchange Act) following the date written notice of the revision is despatched to holders of Celltech Securities. Except with the consent of the Panel, no revision of the Offer may be made or posted to Celltech holders of Securities after 4 July 2004, the date 14 calendar days before the last date on which the Offer can become unconditional.

(c)

The Offer, whether revised or not, will not (except with the consent of the Panel and the SEC) be capable of becoming unconditional after 1.00p.m. (London time) or 8.00 a.m. (New York City time), on 18 July 2004 (or any other time and/or date beyond which UCB has stated that the Offer will not be extended and has not, where permitted, withdrawn that statement), nor of being kept open for acceptances after that time and/or date unless the Offer has previously become unconditional, provided that UCB reserves the right, with the permission of the Panel and the SEC, to extend the Offer to a later time(s) and/or date(s). Except with the consent of the Panel, UCB may not, for the purposes of determining whether the Acceptance Condition has been satisfied, take into account acceptances received or purchases of Celltech Shares (including Celltech Shares represented by Celltech ADSs) made after 1.00 p.m. (London time), 8.00 a.m. (New York City time) on 18 July 2004 (or any other time(s) and/or date(s) beyond which UCB has stated that the Offer will not be extended (and has not, where permitted, withdrawn that statement) or, if the Offer is so extended, such later time(s) and/or date(s) as UCB, with the permission of the Panel, may determine.

(d)

If the Offer becomes unconditional in all respects it will remain open for acceptance for the Subsequent Offer Period which shall be not less than 14 calendar days from the expiry of the Initial Offer Period. If the Offer has become unconditional and it is stated by or on behalf of UCB that the Offer will remain open until further notice, then not less than 14 calendar days' notice in writing will be given to those holders of Celltech Securities who have not accepted the Offer prior to the closing of the Subsequent Offer Period.

(e)

If a competitive situation arises after UCB has made a "no extension" statement and/or a "no increase" statement (as referred to in the City Code) in connection with the Offer, UCB may, if it specifically reserves the right to do so at the time such statement is made (or otherwise with the consent of the Panel), choose not to be bound by or withdraw such statement and be free to revise and/or extend the Offer provided in each circumstance it complies with the requirements of the City Code and the Exchange Act and, in particular, that:

(i)

it announces the withdrawal as soon as possible and in any event within four Business Days of the date of the firm announcement of the competing offer or other competitive situation; and

(ii)

it notifies holders of Celltech Securities to that effect in writing at the earliest opportunity or, in the case of holders of Celltech Securities with registered addresses outside the United Kingdom or whom UCB knows to be nominees, custodians or trustees holding Celltech Shares (including

33

Celltech Shares represented by Celltech ADSs) for such persons, by announcement in the United Kingdom and the United States.

UCB may choose not to be bound by a "no increase" and/or "no extension" statement if, having reserved the right to do so, it posts an increased or improved offer (either as to the value or form of the consideration or otherwise) which is recommended for acceptance by the Celltech Board, or in other circumstances permitted by the Panel.

(f)

For the purposes of determining whether the Acceptance Condition has been satisfied, UCB will not be bound (unless otherwise required by the Panel) to take into account any Celltech Shares (including Celltech Shares represented by Celltech ADSs) which have been issued or unconditionally allotted or which arise as the result of the exercise of subscription or conversion rights before that determination takes place unless written notice containing relevant details of the allotment, issue, subscription or conversion has been received from Celltech or its agents before that time by UCB or Capita IRG Plc on behalf of UCB at The Registry, 34 Beckenham Road, Beckenham, Kent BR3 4TH, (in the case of Celltech Shares) or the Tender Agent at the relevant address set forth in the Letter of Transmittal (in the case of Celltech ADSs). Notification by telex or facsimile or other electronic transmissions or copies will not be sufficient.

(g)

In accordance with applicable SEC policy, at least five US business days prior to any reduction in the percentage of Celltech Shares (including Celltech Shares represented by Celltech ADSs) required to satisfy the Acceptance Condition, UCB will announce that it is to exercise the right so to reduce such percentage. The announcement will be made through a press release designed to inform holders of Celltech Securities in the United Kingdom and elsewhere and by placing an advertisement in a newspaper of national circulation in the United States. Such announcement will state the level to which the percentage required to satisfy the Acceptance Condition may be reduced, that such a reduction is possible but that UCB need not declare its actual intentions until it is required to do so under the City Code and will contain language advising holders of Celltech Securities to withdraw their Celltech Shares (including Celltech Shares represented by Celltech ADSs) if their willingness to accept the Offer would be affected by a reduction of the Acceptance Condition. UCB will not make such an announcement unless it believes that there is a significant possibility that sufficient Celltech Shares (including Celltech Shares represented by Celltech ADSs) will be tendered to permit the Acceptance Condition to be satisfied at such reduced level. Holders of Celltech Securities who are not willing to accept the Offer if the Acceptance Condition is reduced to a level lower than 90 per cent. should either not accept the Offer until the Offer has become unconditional in all respects (which may be at a level of acceptances lower than 90 per cent.) or be prepared to withdraw their acceptances promptly following an announcement by UCB of its reservation of the right to reduce the Acceptance Condition. Upon any announcement being made that the percentage of Celltech Shares (including Celltech Shares represented by Celltech ADSs) required to satisfy the Acceptance Condition may be reduced, the Offer shall not be capable of becoming or being declared unconditional in all respects until the expiry of at least five US business days thereafter. Holders of Celltech Securities will be able to accept the Offer for at least five US business days after the reduction of the Acceptance Condition.

2      Announcements

(a)

By 8.00 a.m. (London time) in the United Kingdom and by 8.00 a.m. (New York City time) in the United States on the Business Day (the "relevant day") following the day on which the Offer is due to expire, or becomes unconditional, or is revised or is extended, as the case may be (or such later time or date as the Panel may agree), UCB will make an appropriate announcement in the United Kingdom and the United States and simultaneously inform a Regulatory Information Service and the Dow Jones News Service of the position. Such announcement will also state (unless otherwise permitted by the Panel) the total number of Celltech Shares (including Celltech Shares represented by Celltech ADSs) and rights over Celltech Shares (including Celltech Shares represented by Celltech ADSs) (as nearly as practicable):

(i)

for which acceptances of the Offer have been received (showing the extent, if any, to which such acceptances have been received from persons acting (or deemed to be acting) in concert with UCB);

(ii)

acquired or agreed to be acquired by or on behalf of UCB or any person acting (or deemed to be acting) in concert with UCB during the Offer Period; and

34

(iii)

held by or on behalf of UCB or any person acting (or deemed to be acting) in concert with UCB prior to the Offer Period,

and will specify the percentage of the Celltech Shares (including the Celltech Shares represented by Celltech ADSs) represented by each of these figures. Any decision to extend the time and/or date by which the acceptance condition has to be satisfied may be made at any time up to, and will be announced not later than, 8.00 a.m. (London Time) in the United Kingdom and by 8.00 a.m. (New York City time) in the United States on the relevant day (or such later time and/or date as the Panel may agree). The announcement will also state the next expiry time and date unless the Offer is unconditional, in which case it may instead state that the Offer will remain open until further notice. In computing the number of Celltech Shares and Celltech ADSs represented by acceptances and purchases, there may be included or excluded for announcement purposes, subject to paragraph 7(f) below, acceptances and purchases not in all respects in order or subject to verification.

(b)

In this Appendix I, references to the making of an announcement or the giving of notice by or on behalf of UCB include the release of an announcement by public relations consultants or by Lazard on behalf of UCB to the press and the delivery by hand or telephone, telex or facsimile transmission or other electronic transmission of an announcement to a Regulatory Information Service and the Dow Jones News Service. An announcement made otherwise than to a Regulatory Information Service will be notified simultaneously to a Regulatory Information Service and the Dow Jones News Service (unless the Panel otherwise agrees).

3      Rights of withdrawal

(a)

Unless otherwise agreed with the Panel and the SEC, UCB will only make an announcement that the Acceptance Condition has been satisfied, if all other Conditions are also satisfied, fulfilled or to the extent permitted, waived. If UCB, having announced the Offer to be unconditional, fails by 3.30 p.m. (London time), 10.30 a.m. (New York City time), on the relevant day (as defined in paragraph 2(a) of this Part B) (or such later time and/or date as the Panel may agree) to comply with any of the other relevant requirements specified in paragraph 2(a) of this Part B, an accepting holder of Celltech Securities may (unless the Panel otherwise agrees) immediately after that time withdraw his acceptance of the Offer by written notice signed by the accepting holder of Celltech Securities (or his agent duly appointed in writing and evidence of whose appointment, in a form reasonably satisfactory to UCB, is produced with the notice) given by post or by hand to Capita IRG Plc, Corporate Actions, PO Box 166, The Registry, 34 Beckenham Road, Beckenham, Kent BR3 4TH or to the Tender Agent, as applicable. Subject to paragraph 1(c) of this Part B, this right of withdrawal may be terminated not less than eight calendar days after the relevant day by UCB confirming, if that be the case, that the Offer is still unconditional and complying with the other relevant requirements specified in paragraph 2(a) of this Part B. If any such confirmation is given, the first period of 14 calendar days referred to in paragraph 1(d) of this Part B will run from the date of that confirmation and compliance.

(b)

Celltech Securities in respect of which acceptances have been received may be withdrawn pursuant to the procedures set out below at any time from the date of this document until the latest time specified for receipt of acceptances on the last day of the Initial Offer Period and in certain other circumstances described below. Celltech Securities in respect of which acceptances have been received during the Initial Offer Period and not validly withdrawn during the Initial Offer Period, and Celltech Securities in respect of which acceptances have been received during the Subsequent Offer Period, may not be withdrawn, except in certain limited circumstances described herein.

(c)

If a "no extension" and/or "no increase" statement is withdrawn in accordance with paragraph 1(e) of this Part B, any acceptance made by a Celltech Shareholder after the date of that statement may be withdrawn thereafter in the manner referred to in paragraph 3(a) above for a period of eight calendar days following the date on which the notice of the withdrawal is posted to holders of Celltech Securities.

(d)

Except as provided by this paragraph 3, acceptances and elections made in accordance with this Appendix I will be irrevocable.

(e)

In this paragraph 3, "written notice" (including any letter of appointment, direction or authority) means notice in writing bearing the original signature(s) of the relevant accepting holders of Celltech Securities or his/their agent(s) duly appointed in writing (evidence of whose appointment satisfactory to UCB is produced with the notice). Telex, email, facsimile or other electronic transmissions or

35

copies will not be sufficient. No notice which is post-marked in, or otherwise appears to UCB or its agents to have been sent from a Restricted Jurisdiction will be treated as valid.

(f)

To be effective, a written notice of withdrawal must be received on a timely basis by the party (either the Receiving Agent or the Tender Agent) to whom the acceptance was originally sent at their respective address and must specify the name of the person who has tendered the Celltech Shares or Celltech ADSs, the number of Celltech Shares or Celltech ADSs to be withdrawn and (if Celltech ADSs have been tendered) the name of the registered holder, if different from the name of the person whose acceptance is to be withdrawn.

(g)

In respect of Celltech ADSs, if Celltech ADRs have been delivered or otherwise identified to the Tender Agent, then, prior to the physical release of such Celltech ADRs, the serial numbers shown on such Celltech ADRs must be submitted and, unless the Celltech ADSs evidenced by such Celltech ADRs have been delivered by an Eligible Institution, the signatures on the notice of withdrawal must be guaranteed by an Eligible Institution. If Celltech ADSs evidenced by Celltech ADRs have been delivered pursuant to the procedures for book-entry transfer set out in paragraph 2(c) of Part C, any notice of withdrawal must specify the name and number of the account at the appropriate Book-Entry Transfer Facility to be credited with the withdrawn Celltech ADSs and must otherwise comply with such Book-Entry Transfer Facility's procedures.

(h)

All questions as to the validity (including time of receipt) of any notice of withdrawal will be determined by UCB whose determination (except as required by the Panel) will be final and binding. None of UCB, Celltech, Lazard, the Tender Agent, the Receiving Agent or any of their affiliates or any other person will be under any duty to give notification of any defects or irregularities in any notice of withdrawal or incur any liability for failure to give such notification or for any determination under this paragraph.

4      The Loan Note Alternative

(a)

The Loan Note Alternative is conditional upon all of the Conditions becoming or being declared satisfied, fulfilled or, to the extent permitted, waived. No Loan Notes will be issued unless valid elections for the Loan Note Alternative are received on or before the date on which the Offer becomes or is declared unconditional in all respects for at least £5 million nominal value of Loan Notes. If insufficient elections are received, Celltech Shareholders who validly accept the Offer and elect for the Loan Note Alternative will instead receive cash in accordance with the terms of the Offer. Subject as aforesaid, the Loan Note Alternative will remain open as long as the Offer is open for acceptance.

(b)

No election for the Loan Note Alternative will be valid unless both a valid acceptance of the Offer and a valid election for the Loan Note Alternative, duly complete in all respects and accompanied by, if appropriate, all relevant share certificates and/or other document(s) of title, are duly received by the time and date on which the Loan Note Alternative closes.

(c)

If any acceptance of the Offer which includes an election for the Loan Note Alternative is not, or is not deemed to be, valid or complete in all respects at such time, such election shall for all purposes be void and the holder(s) of Celltech Shares purporting to make such election shall not, for any purpose, be entitled to receive the Loan Note Alternative, but any such acceptance which is otherwise valid shall be deemed to be an acceptance of the Offer (without the Loan Note Alternative) for the number of Celltech Shares which are the subject of the acceptance and the holder(s) of Celltech Shares will, on the Offer becoming unconditional, receive the cash consideration due under the Offer.

(d)

The insertion of a number in Box 3 on the Form of Acceptance shall, subject to the other terms of the Offer, be treated in respect of that number of Celltech Shares, as an election for the Loan Note Alternative.

(e)

An election for the Loan Note Alternative will not be valid unless the Form of Acceptance is completed correctly in all respects and is received in accordance with paragraph 8 below.

(f)

The Loan Notes will be issued in multiples of £1 and fractional entitlements will be disregarded.

(g)

The Loan Note Alternative is not available to any Celltech Shareholder who is a US person or to holders of Celltech ADSs or to certain other Overseas Shareholders as referred to in paragraph 6(h) of this Part B.

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5      Revised Offer

(a)

Although no such revision is envisaged, if the Offer (in its original or any previously revised form(s)) is revised (either in its terms or conditions or in the value or form of the consideration offered or otherwise), and any such revised Offer represents on the date on which the revision is announced (on such basis as Lazard may consider appropriate) an improvement (or no diminution) in the value of the consideration of the Offer as so revised compared with the value of the consideration or terms previously offered, or in the overall value received by a holder of Celltech Securities (under or in consequence of the Offer or otherwise), the benefit of the revised Offer will, subject to paragraphs 5(c), 5(d) and 6 of this Part B, be made available to any holder of Celltech Securities who has validly accepted the Offer in its original or any previously revised form(s) (a "Previous Acceptor"). The acceptance by or on behalf of a Previous Acceptor of the Offer in its original or any previously revised form(s) shall, subject to paragraphs 5(c), 5(d) and 6 of this Part B, be deemed to be an acceptance of the Offer as so revised and will also constitute an authority to UCB, Lazard or any of their respective directors, authorised representatives and agents as his attorney and/or agent ("attorney"):

(i)

to accept any such revised Offer on behalf of such Previous Acceptor;

(ii)

if such revised Offer includes alternative forms of consideration, to make on his behalf elections for and/or accept such alternative forms of consideration on his behalf in such proportions as such attorney in his absolute discretion thinks fit; and

(iii)

to execute on behalf of and in the name of such Previous Acceptor all such further documents (if any) and take such further actions (if any) as may be required to give effect to such acceptances and/or elections.

In making any such acceptance or making any such election, the attorney will take into account the nature of any previous acceptances and/or elections made by the Previous Acceptor and such other facts or matters as he may reasonably consider relevant.

(b)

UCB and Lazard reserve the right (subject to paragraph 5(a) above) to treat an executed Form of Acceptance or Letter of Transmittal relating to the Offer in its original or any previously revised form(s) which is received (or dated) on or after the announcement or issue of the Offer in any revised form as a valid acceptance of the revised Offer (and, where applicable, a valid election for the alternative form(s) of consideration). Such acceptance will constitute an authority in the terms of paragraph 5(a) above, mutatis mutandis, on behalf of the relevant holder of Celltech Securities.

(c)

The deemed acceptances referred to in this paragraph 5 shall not apply and the authorities conferred by this paragraph 5 shall not be exercised by UCB or Lazard or any of their respective directors, authorised representatives and agents if, as a result thereof, the Previous Acceptor would (on such basis as Lazard may consider appropriate) thereby receive less in aggregate consideration under the revised Offer than he would have received in aggregate consideration as a result of acceptance of the Offer in the form in which it was originally accepted by him or on his behalf.

(d)

The deemed acceptances referred to in this paragraph 5 will not apply and the authorities conferred by this paragraph will be ineffective in the case of a Previous Acceptor who lodges with Capita IRG Plc or the Tender Agent (as appropriate) within 14 calendar days of the posting of the document pursuant to which the revision of the Offer is made available to holders of Celltech Securities, a Form of Acceptance (in the case of Celltech Shares), Letter of Transmittal (in the case of Celltech ADSs) or some other form issued by or on behalf of UCB in which a holder of Celltech Securities validly elects to receive the consideration receivable by him under such revised Offer in some other manner than that set out in his original acceptance.

(e)

If UCB makes a material change in the terms of the Offer or if it waives a material condition of the Offer, UCB will disseminate additional tender offer materials and extend the Offer to the extent required by Rules 14d-4(d), 14d-6(c) and 14e-1 under the Exchange Act. The minimum period during which an offer must remain open following material changes in the terms of the offer, other than a change in price or a change in the percentage of securities sought, will depend upon the facts and circumstances then existing, including the materiality of the changes, but generally will be no less than five US business days. With respect to a change in price or, subject to certain limitations, a change in the percentage of securities sought, a minimum of ten US business days is generally required to allow for adequate dissemination to holders of Celltech Securities. Any reduction in the percentage of

37

Celltech Shares required to satisfy the acceptance condition will be accomplished and announced in the manner described in paragraph 1(g) of this Part B and not in accordance with this paragraph.

6      Overseas Holders of Celltech Securities

(a)

The making of the Offer (including the Loan Note Alternative) in, or to certain persons who are resident in, or citizens or nationals of, jurisdictions outside the United Kingdom or to custodians, nominees of or trustees for such persons, may be prohibited or affected by the laws of the relevant jurisdictions. Overseas Shareholders should inform themselves about and observe any applicable legal requirements. It is the responsibility of any such person wishing to accept the Offer (or the Loan Note Alternative) to satisfy himself as to the full observance of the laws and regulatory requirements of the relevant jurisdiction in connection therewith, including the obtaining of any governmental, exchange control or other consents which may be required, or the compliance with other necessary formalities needing to be observed and the payment of any issue, transfer or other taxes or other requisite payments due in that jurisdiction. Any such Overseas Shareholder will be responsible for the payment of any issue, transfer or other taxes due in that jurisdiction of whomsoever payable and UCB and Lazard and any person acting on their behalf shall be fully indemnified and held harmless by such Overseas Shareholder for any such issue, transfer or other taxes as such person may be required to pay. If you are an Overseas Shareholder and you are in doubt about your position, you should consult your professional adviser in the relevant jurisdiction.

(b)

Unless otherwise determined by UCB, the Offer is not being made, directly or indirectly, in or into any Restricted Jurisdiction or by use of the mails of, or by any means or instrumentality of interstate or foreign commerce of, or any facilities of a national securities exchange of, any of such jurisdictions. Such means or instrumentalities include, but are not limited to, facsimile transmission, telex, telephone and the internet.

(c)

Copies of this document, the Acceptance Forms and any related document(s) are not being (unless otherwise determined by UCB), and must not be, mailed or otherwise distributed or sent in or into any Restricted Jurisdiction including to Celltech Shareholders with registered addresses in these jurisdictions or to persons whom UCB knows to be nominees, custodians or trustees holding Celltech Shares for such persons. Persons receiving such documents (including, without limitation, custodians, nominees and trustees) must not distribute, send or mail them in, into or from a Restricted Jurisdiction or use any such instrumentality for any purpose, directly or indirectly, in connection with the Offer, and doing so may render invalid any related purported acceptance of the Offer. Persons wishing to accept the Offer must not use the mails or any such other instrumentality of any Restricted Jurisdiction and other documents relating to the Offer must not be postmarked in, or otherwise despatched, from a Restricted Jurisdiction and all acceptors must provide addresses outside a Restricted Jurisdiction for the receipt of the consideration to which they are entitled under the Offer and which is despatched by post pursuant to paragraph 1(e)(ii) of Part C of this Appendix I or for the return of the Acceptance Forms and (in relation to Celltech Shares in certificated form) any Celltech share certificate(s) and/or other document(s) of title.

(d)

Subject as provided below, a Celltech Shareholder will be deemed NOT to have accepted the Offer if:

(i)

he puts "NO" in Box 6 of the Form of Acceptance and thereby does not make the representations and warranties set out in paragraph 1(b) of Part C of this Appendix I;

(ii)

Box 1 of the Form of Acceptance contains an address in a Restricted Jurisdiction or the relevant Celltech Shareholder has a registered address in a Restricted Jurisdiction and in either case he does not insert in Box 7 of the Form of Acceptance the name and address of a person or agent outside a Restricted Jurisdiction to whom he wishes the consideration to which he is entitled under the Offer to be sent, subject to the provisions of this paragraph 6(d) and applicable laws;

(iii)

he inserts in Box 8 of the Form of Acceptance a telephone number in a Restricted Jurisdiction for use in the event of queries; or

(iv)

a Form of Acceptance received from him is received in an envelope postmarked in, or otherwise appears to UCB or its agents to have been sent from, a Restricted Jurisdiction.

UCB reserves the right, in its sole discretion, to investigate, in relation to any acceptance, whether the representations and warranties set out in paragraph 1(b) of Part C of this Appendix I could have been truthfully given by the relevant holder of Celltech Securities and, if such investigation is made and as a

38

result UCB determines that such representations and warranties could not have been so given, such acceptance shall not be valid.

(e)

If, notwithstanding the restrictions described above, any person (including, without limitation, custodians, nominees and trustees) whether pursuant to a contractual or legal obligation or otherwise forwards this document, the Acceptance Forms or any related offer document in, into or from a Restricted Jurisdiction or uses the mails or any means or instrumentality (including, without limitation, facsimile transmission, email, telex, telephone and the internet) of interstate or foreign commerce of, or any facilities of a national securities exchange of, any such jurisdiction in connection with such forwarding, such person should:

(i)

inform the recipient of such fact;

(ii)

explain to the recipient that such action may invalidate any purported acceptance by the recipient; and

(iii)

draw the attention of the recipient to this paragraph 6.

(f)

Notwithstanding anything to the contrary contained in this document or the Acceptance Forms, UCB and Lazard may make the Offer (with or without giving effect to the foregoing paragraphs of this paragraph 6) in a Restricted Jurisdiction or in any other jurisdiction if requested to do so by or on behalf of a person if UCB is satisfied, in that particular case, that to do so would not constitute a breach of any securities or other relevant legislation of any such jurisdiction or pursuant to an exemption under, or in accordance with, applicable law in such jurisdictions and, in this connection, the provisions of paragraph 1(c) of Part C of this Appendix I will be varied accordingly.

(g)

The provisions of this paragraph 6 supersede any terms of the Offer inconsistent with them. The provisions of this paragraph 6 and/or any other terms of the Offer relating to Overseas Shareholders may be waived, varied or modified as regards specific holder(s) of Celltech Securities or on a general basis by UCB in its absolute discretion.

(h)

The Loan Notes have not been, and will not be, listed on any stock exchange and have not been and will not be registered under the Securities Act, or under any relevant securities laws of any state or other jurisdiction of the United States and the relevant clearances have not been, and will not be, obtained from the regulatory authority of any province or territory of Canada. In addition, no prospectus in relation to the Loan Notes has been, or will be, lodged with, or registered by, the Australian Securities and Investments Commission and no steps have been taken, nor will any be taken, to enable the Loan Notes to be offered in compliance with applicable securities laws of Japan or Belgium. Accordingly, unless an exemption under such relevant laws is available, the Loan Notes may not be offered, sold, resold or delivered, directly or indirectly, in, into or from the United States or a Restricted Jurisdiction or any other jurisdiction in which an offer of Loan Notes would constitute a violation of relevant laws or require registration of the Loan Notes, or to or for the benefit of any US person or resident of a Restricted Jurisdiction or any other such jurisdiction. If, in respect of a Form of Acceptance from any Celltech Shareholder, the holder is unable to make the representations and warranties set out in paragraphs 1(b) and 1(q) of Part C of this Appendix I, UCB reserves the right, in its absolute discretion, to ignore any election in that Form of Acceptance to receive Loan Notes and to treat it instead as an acceptance of the Offer for cash.

(i)

The Offer in the United States is made solely by UCB, and neither Lazard nor any of its affiliates is making the Offer in the United States. Lazard Frères & Co. LLC is acting as dealer manager of the Offer in the United States.

7      General

(a)

The Offer will lapse unless all conditions relating to the Offer have been fulfilled or, where appropriate, have been and continue to be satisfied or have been waived by 1.00 p.m. (London time), 8.00 a.m. (New York City time), on 18 July 2004 or such later date as UCB, with the consent of the Panel and the SEC, may decide.

(b)

If the Offer lapses or is withdrawn, it will cease to be capable of further acceptance and accepting holders of Celltech Securities, UCB and Lazard will cease to be bound by Acceptance Forms submitted before the time the Offer lapses or is withdrawn.

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(c)

If sufficient acceptances under the Offer are received and/or sufficient Celltech Shares are otherwise acquired, UCB intends to apply the provisions of sections 428 to 430F of the Companies Act to acquire compulsorily any outstanding Celltech Shares (including Celltech Shares represented by Celltech ADSs). In respect of Celltech ADSs validly tendered pursuant to a Letter of Transmittal, UCB shall, insofar as it may determine, be deemed for the purposes of the Acceptance Condition or generally to have agreed to acquire such Celltech ADSs and the Celltech Shares represented by such Celltech ADSs.

Holders of Celltech Securities do not have appraisal rights as a result of the Offer. However, in the event that compulsory acquisition procedures referred to above are available to UCB, holders of Celltech Securities whose Celltech Securities have not been purchased pursaunt to the Offer may have certain rights to object under section 430C of the Companies Act.

(d)

Furthermore, UCB intends to procure that Celltech applies to the UK Listing Authority for the cancellation of the listing of Celltech Shares on the Official List and to the London Stock Exchange for the cancellation of trading of Celltech Shares. Such cancellations will take effect no earlier than 20 Business Days after the date on which the Offer becomes or is declared unconditional in all respects. UCB also intends to procure that Celltech applies for de-listing of the Celltech ADSs from the New York Stock Exchange.

(e)

Except with the consent of the Panel, settlement of the consideration to which any holder of Celltech Securities is entitled under the Offer will be implemented in full in accordance with the terms of the Offer without regard to any lien, right of set-off, counterclaim or other analogous right to which UCB may otherwise be, or claim to be, entitled as against such holder of Celltech Securities and will be effected:

(i)

in the case of acceptances received, complete in all respects (including the relevant transfer to escrow or (as applicable) receipt of relevant share certificate(s), Celltech ADRs and/or other documents of title or indemnities satisfactory to UCB), by the date on which the Offer becomes or is declared unconditional in all respects, within 14 calendar days of such date; or

(ii)

in the case of acceptances of the Offer received, complete in all respects, after the date on which the Offer becomes or is declared unconditional in all respects, but while it remains open for acceptance, within 14 calendar days of such receipt.

Settlement will be made in the manner outlined in paragraph 20 of the letter from Lazard set out in Part II of this document. Unless otherwise determined by UCB, no consideration will be sent to an address in a Restricted Jurisdiction and no Loan Notes will be sent to an address in the United States or to a US person.

(f)

Notwithstanding the right reserved by UCB and Lazard to treat a Form of Acceptance or Letter of Transmittal as valid (even though not entirely in order or not accompanied by the relevant Celltech Share certificate(s), Celltech ADRs and/or other documents of title or indemnities satisfactory to UCB, or not accompanied by the relevant transfer to escrow), except as otherwise agreed with the Panel:

(i)

an acceptance of the Offer will only be counted towards fulfilling the Acceptance Condition if the requirements of Note 4 and, if applicable, Note 6 on Rule 10 of the City Code are satisfied in respect of it;

(ii)

a purchase of Celltech Shares (including Celltech Shares represented by Celltech ADSs) by UCB or its nominee(s) (or, if relevant, any person acting in concert with UCB, or its nominee(s)) will only be counted towards fulfilling the Acceptance Condition if the requirements of Note 5 and, if applicable, Note 6 on Rule 10 of the City Code are satisfied in respect of it; and

(iii)

the Offer will not become unconditional unless Capita IRG Plc has issued a certificate to UCB and/or Lazard or their respective agents stating the number of Celltech Shares and Celltech ADSs in respect of which acceptances have been received which comply with sub-paragraph (i) above and the number of Celltech Shares and Celltech ADSs otherwise acquired, whether before or during the Offer Period, which comply with sub-paragraph (ii) above. Lazard will send a copy of such certificate to the Panel and to Morgan Stanley and JPMorgan as soon as possible after it is issued.

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(g)

The terms, provisions, instructions and authorities contained in or deemed to be incorporated in the Acceptance Forms constitute part of the terms of the Offer. Words and expressions defined in this document have the same meanings when used in the Acceptance Forms, unless the context otherwise requires. The provisions of this Appendix I shall be deemed to be incorporated into and form part of the Acceptance Forms.

(h)

All references in this document and in the Acceptance Forms to 17 June 2004 will (except in the definition of Offer Period and where the context otherwise requires) be deemed, if the expiry date of the Offer is extended, to refer to the expiry date of the Offer as so extended.

(i)

References in paragraph 6 of this Part B and in Part C of this Appendix I to a holder of Celltech Securities will include references to the person or persons executing a Form of Acceptance or Letter of Transmittal and, in the event of more than one person executing a Form of Acceptance or Letter of Transmittal, such paragraphs will apply to them jointly and severally.

(j)

Any omission or failure to despatch this document, the Form of Acceptance or Letter of Transmittal, any other documents relating to the Offer and/or any notice required to be despatched under the terms of the Offer to, or any failure to receive the same by, any person to whom the Offer is made, or should be made, will not invalidate the Offer in any way.

(k)

UCB and Lazard reserve the right to treat acceptances of the Offer as valid if received by or on behalf of either of them at any place or places determined by them otherwise than as set out in this document or the Acceptance Forms.

(l)

No acknowledgement of receipt of any Form of Acceptance or Letter of Transmittal, transfer by means of CREST, share certificate(s) or other document(s) of title will be given by, or on behalf of UCB. All communications, notices, certificates, documents of title and remittances to be delivered by, or sent to or from, holders of Celltech Securities (or their designated agent(s)) will be delivered by or sent to or from them (or their designated agent(s)) at their own risk.

(m)

For those persons to whom the Offer is made or should be made but to whom this document, the Acceptance Forms or any related documents may not be despatched, such persons may collect copies of these documents from Capita IRG Plc (in the case of Celltech Shares) and the Tender Agent or the US Information Agent (in the case of Celltech ADSs) at the relevant addresses.

(n)

UCB and Lazard reserve the right to notify any matter including the making of the Offer to all or any holder of Celltech Securities with a registered address outside the United Kingdom and the United States or whom UCB knows to be a custodian, trustee or nominee holding Celltech Shares (including Celltech Shares represented by Celltech ADSs) for persons who are citizens, residents or nationals of jurisdictions outside the United Kingdom and the United States, by announcement in the United Kingdom to a Regulatory Information Service and in the United States to the Dow Jones News Service or in any other appropriate manner, or by paid advertisement in a daily newspaper published and circulated in the United Kingdom and the United States, in which event such notice will be deemed to have been sufficiently given, notwithstanding any failure by any such holder(s) of Celltech Securities to receive or see such notice, and all references in this document to notice in writing by or on behalf of UCB will be construed accordingly.

(o)

The Offer is made at 3.00 p.m. (London time), 10.00 a.m. (New York City time) on 19 May 2004 and is capable of acceptance from and after that time. The Offer is being made by means of this document and will be communicated to holders of Celltech Securities by means of an advertisement proposed to be published in the London edition of the Financial Times and the US national edition of The Wall Street Journal.

(p)

If the Offer does not become unconditional in all respects:

(i)

in respect of Celltech Shares held in certificated form and Celltech ADSs, the Form of Acceptance, relevant share certificates, Celltech ADRs and/or other documents of title will be returned by post (or such other method as may be approved by the Panel) within 14 calendar days of the Offer lapsing or being withdrawn to the person or agent whose name and address outside a Restricted Jurisdiction set out in the relevant box in the Form of Acceptance or Letter of Transmittal (as applicable) or, if none is set out, to the first-named holder at his registered address outside a Restricted Jurisdiction (no such documents will be sent to an address in a Restricted Jurisdiction);

41

(ii)

in respect of Celltech Shares held in uncertificated form, Capita IRG Plc will, immediately after the lapsing or withdrawal of the Offer (or within such longer period as the Panel may permit, not exceeding 14 calendar days of the lapsing or withdrawal of the Offer), give instructions to CRESTCo to transfer all Celltech Shares held in escrow balances and in relation to which it is the Offer Escrow Agent for the purposes of the Offer to the original available balances of Celltech Shareholders concerned; and



(iii)

in respect of Celltech ADSs delivered by book-entry transfer into the Tender Agent's account at a Book-Entry Transfer Facility, Celltech ADSs will be credited to an account maintained at the appropriate Book-Entry Transfer Facility.

(q)

All powers of attorney, appointments of agents and authorities conferred by this Appendix I or in the Form of Acceptance are given by way of security for the performance of the obligations of Celltech Shareholders concerned and are irrevocable in accordance with section 4 of the Powers of Attorney Act 1971 except in the circumstances where the donor of such power of attorney or authority or appointer is entitled to withdraw his acceptance in accordance with paragraph 3 of this Part B and duly does so.

(r)

In relation to any acceptance of the Offer in respect of a holding of Celltech Shares which are in uncertificated form, UCB reserves the right to make such alterations, additions or modifications as may be necessary or desirable to give effect to any purported acceptance of the Offer, whether in order to comply with the facilities or requirements of CREST or otherwise, provided any such alterations, additions or modifications are consistent with the requirements of the City Code or are otherwise made with the consent of the Panel.

(s)

Neither UCB nor Lazard nor any agent or director of UCB or Lazard, nor any person acting on behalf of any of them, shall have any liability to any person for any loss or alleged loss arising from any decision as to the treatment of acceptances of the Offer or otherwise in connection therewith.

(t)

The Offer, all Forms of Acceptance and all acceptances will be governed by and construed in accordance with English law. Execution by or on behalf of a Celltech Shareholder or a holder of Celltech ADSs of an Acceptance Form constitutes his irrevocable submission to the exclusive jurisdiction of the courts of England in relation to all matters arising in connection with the Offer.

(u)

Subject to the Offer becoming or being declared unconditional in all respects, UCB intends to procure that Celltech applies to the UKLA for the official listing and to the London Stock Exchange for the admission to trading of the Celltech Shares to be cancelled. It is anticipated that such cancellation will take effect no earlier than 20 Business Days after the Offer becomes unconditional in all respects. UCB also intends to procure that Celltech applies for de-listing of the Celltech ADSs from the New York Stock Exchange.

8      Procedures for Tendering Celltech Shares

(a)

Holders of Celltech Shares will have received with this document a Form of Acceptance. This section should be read together with the Form of Acceptance. The provisions of this section shall be deemed to be incorporated in, and to form a part of, the Form of Acceptance. The instructions printed on the Form of Acceptance shall be deemed to form part of the terms of the Offer.

If a holder of Celltech Shares holds Celltech Shares in both certificated and uncertificated form, he should complete a separate Form of Acceptance for each holding. Similarly, such holder should complete a separate Form of Acceptance for Celltech Shares held in uncertificated form, but under different member account IDs, and for Celltech Shares held in certificated form, but under different designations. Please contact the Receiving Agent if you require any additional Forms of Acceptance.

(b)

If a holder of Celltech Securities is electing for the Loan Note Alternative he must not be in the United States or be a resident of a Restricted Jurisdiction, must not be a US person and must not accept the Offer and elect for Loan Notes with a view to the offer, sale or delivery directly or indirectly, of such Loan Notes in or into the United States or a Restricted Jurisdiction and he must not hold or acquire any Loan Notes for any other person who he has reason to believe is purchasing for the purpose of that offer, sale or delivery.

(c)

To accept the Offer, any Celltech Shareholder, including any person in the United States who holds Celltech Shares, wishing to accept the Offer in respect of all or any portion of such holder's Celltech Shares, should complete Box 2 and, sign Box 4 in accordance with the instructions printed on it and, if

42

applicable, complete Boxes 3, 7 and 8 and, if such holder's Celltech Shares are in CREST, Box 5. All Celltech Shareholders who are individuals should sign the Form of Acceptance in the presence of a witness, who should also sign Box 4 in accordance with the instructions printed on it. Unless witnessed, an acceptance by a Celltech Shareholder who is an individual will not be valid.

(d)

An accepting Celltech Shareholder should return the completed, signed and witnessed (if applicable) Form(s) of Acceptance, whether or not such Celltech Shares are in CREST, to the Receiving Agent. The completed Form(s) of Acceptance, together, if such holder's Celltech Shares are held in certificated form, with his share certificate(s) and/or other document(s) of title, must be lodged with the Receiving Agent, as soon as possible, but in any event so as to arrive not later than 3.00 p.m. (London time), 10.00 a.m. (New York City time), on 17 June 2004. If you have any questions as to how to complete the Form(s) of Acceptance, please contact the Helpline. A Form of Acceptance contained in an envelope postmarked in a Restricted Jurisdiction or otherwise appearing to UCB or its agents to have been sent from a Restricted Jurisdiction may be rejected as invalid.

(e)

If Celltech Shares are held in uncertificated form, the holder should insert in Box 5 of the Form of Acceptance the participant ID and member account ID under which such Celltech Shares are held by him in CREST and otherwise complete and return the Form of Acceptance as described above. In addition, such holders should take (or procure to be taken) the action set out below to transfer the Celltech Shares in respect of which he wishes to accept the Offer to an escrow balance, specifying the Receiving Agent (in its capacity as a CREST participant under the participant ID referred to below) as the escrow agent, as soon as possible, but in any event so that the transfer to escrow settles not later than 3.00 p.m. (London time), 10.00 a.m. (New York City time), on 17 June 2004.

(f)

If the Celltech Shareholder is a CREST sponsored member, he should refer to his CREST sponsor before taking any action. Such holder's sponsor will be able to confirm details of his participant ID and the member account ID under which his Celltech Shares are held. In addition, only his CREST sponsor will be able to send the TTE Instruction to CRESTCo in relation to his Celltech Shares.

(g)

A holder of Celltech Shares should send (or, if he is a CREST sponsored member, procure that his CREST sponsor sends) a TTE Instruction to CRESTCo, which must be properly authenticated in accordance with CRESTCo's specifications and which must contain, in addition to the other information that is required for a TTE Instruction to settle in CREST, the following details:

(i)

the Corporate Action ISIN. This is GB0001822765;

(ii)

the number of Celltech Shares to be transferred to an escrow balance;

(iii)

the participant ID of such Celltech Shareholder. This must be the same participant ID as the participant ID that is inserted in Box 5 of the Form of Acceptance;

(iv)

the member account ID of such Celltech Shareholder. This must be the same member account ID as the member account ID that is inserted in Box 5 of the Form of Acceptance;

(v)

the participant ID of the escrow agent, Capita IRG Plc, in its capacity as a CREST receiving agent. This is RA10;

(vi)

the member account ID of the escrow agent. This is CELLTECH;

(vii)

the Form of Acceptance reference number. This is the number that appears next to Box 5 on page 3 of the Form of Acceptance. This reference number should be inserted in the first eight characters of the shared note field on the TTE Instruction. Such insertion will enable the Receiving Agent to match the transfer to escrow to your Form of Acceptance. The Celltech Shareholder should keep a separate record of this Form of Acceptance reference number for future reference;

(viii)

the intended settlement date. This should be as soon as possible and in any event not later than 3.00 p.m. (London time), 10.00 a.m. (New York City time), on 17 June 2004;

(ix)

the Corporate Action Number for the Offer. This is allocated by CRESTCo and can be found by viewing the relevant Corporate Action Details in CREST; and

(x)

input with Standard Delivery instruction of 80.

(h)

After settlement of the TTE Instruction, a Celltech Shareholder will not be able to access the Celltech Shares concerned in CREST for any transaction or charging purposes. If all of the conditions are

43

either satisfied, fulfilled or, to the extent permitted, waived, the escrow agent will transfer the Celltech Shares concerned to itself in accordance with paragraph 1(d)(i) of Part C of this Appendix.

(i)

A Celltech Shareholder holding Celltech Shares in uncertificated form is recommended to refer to the CREST Manual published by CRESTCo for further information on the CREST procedures outlined above. For ease of processing, such holder is requested, wherever possible, to ensure that a Form of Acceptance relates to only one transfer to escrow.

(j)

If no Form of Acceptance reference number, or an incorrect Form of Acceptance reference number, is included on the TTE Instruction, UCB may treat any amount of Celltech Shares transferred to an escrow balance in favour of the escrow agent specified above from the participant ID and member account ID identified in the TTE Instruction as relating to any Form(s) of Acceptance which relate(s) to the same member account ID and participant ID (up to the amount of Celltech Shares inserted or deemed to be inserted on the Form(s) of Acceptance concerned).

(k)

A Celltech Shareholder holding Celltech Shares in uncertificated form should note that CRESTCo does not make available special procedures, in CREST, for any particular corporate action. Normal system timings and limitations will therefore apply in connection with a TTE Instruction and its settlement. Such holder should therefore ensure that all necessary action is taken by him (or by his CREST sponsor) to enable a TTE Instruction relating to his Celltech Shares to settle prior to 3.00 p.m. (London time), 10.00 a.m. (New York City time), on 17 June 2004. In this connection, such holder is referred in particular to those sections of the CREST Manual concerning practical limitations of the CREST system and timings.

(l)

UCB will make an appropriate announcement if any of the details contained in this paragraph 8 alter for any reason.

(m)

Normal CREST procedures (including timings) apply in relation to any Celltech Shares that are, or are to be, converted from uncertificated to certificated form, or vice versa, during the course of the Offer (whether any such conversion arises as a result of a transfer of Celltech Shares or otherwise). Celltech Shareholders who are proposing so to convert any Celltech Shares are recommended to ensure that the conversion procedures are implemented in sufficient time to enable the person holding or acquiring the Celltech Shares as a result of the conversion to take all necessary steps in connection with an acceptance of the Offer (in particular, as regards delivery of share certificate(s) and/or other document(s) of title or transfers to an escrow balance as described above) prior to 3.00 p.m. (London time), 10.00 a.m. (New York City time), on 17 June 2004.

(n)

If the share certificate(s) and/or other document(s) of title is/are not readily available or is/are lost, the Form of Acceptance should nevertheless be completed, signed and sent as stated above to the Receiving Agent so as to be received as soon as possible, but in any event no later than 3.00 p.m. (London time), 10.00 a.m. (New York City time), on 17 June 2004, together with any share certificate(s) and/or other document(s) of title that is/are available, accompanied by a letter stating that the balance will follow or that the accepting holder has lost one or more of his share certificate(s) and/or other documents of title. If the share certificate(s) and/or other document(s) of title are lost, the accepting holder should request the Registrar to send him a letter of indemnity for completion in accordance with the instructions given. When completed, the letter of indemnity must be lodged with the Receiving Agent in accordance with the instructions given, in support of the Form of Acceptance. UCB may treat as invalid, to the extent that it so determines in its absolute discretion, any acceptance from the Tender Agent for the Celltech ADR programme which UCB has reason to believe has not been properly authorised by the relevant ADS holder and/or is inconsistent with any acceptance received from an ADS holder.

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Part C—Acceptance Forms

1      Form of Acceptance for Celltech Shareholders

        Each holder of Celltech Shares who executes and lodges or has executed and lodged on his behalf a Form of Acceptance with the Receiving Agent, subject to the rights of withdrawal set out in this document, irrevocably (and so as to bind himself, his heirs, successors and assigns and his personal or legal representatives):

(a)

(i)    accepts the Offer in respect of the number of Celltech Shares inserted or deemed to be inserted in Box 2 of the Form of Acceptance;

(ii)

elects under the Loan Note Alternative in respect of the number of Celltech Shares inserted or deemed to be inserted in Box 3 of the Form of the Acceptance; and

(iii)

agrees to execute any further documents and give any further assurances which may be required to enable UCB to obtain the full benefit of Part B and paragraph 1 of this Part C and/or to perfect any of the authorities expressed to be given hereunder, in each case on and subject to the terms and conditions set out or referred to in this document and the Form of Acceptance;

(b)

represents and warrants to UCB and Lazard and the Receiving Agent that, unless "NO" is put in Box 6 of the Form of Acceptance:

(i)

he has not received or sent copies or originals of this document, the Form of Acceptance or any related offering documents in, into or from a Restricted Jurisdiction;

(ii)

he has not used in connection with the Offer or the execution or delivery of the Form of Acceptance, directly or indirectly, the mails of, or any means or instrumentality (including, without limitation, e-mail, facsimile transmission, telex and telephone) of interstate or foreign commerce of, or any facility of a national securities exchange of a Restricted Jurisdiction;

(iii)

he is accepting the Offer from outside a Restricted Jurisdiction; and

(iv)

he is not an agent or fiduciary acting on a non-discretionary basis for a principal, unless such agent or fiduciary is an authorised employee of such principal or such principal has given all instructions with respect to the Offer from outside a Restricted Jurisdiction;

(c)

appoints any director of, or any person authorised by, UCB or Lazard as his agent and/or attorney (subject to the Offer becoming unconditional in all respects and him not having validly withdrawn his acceptance) with an irrevocable instruction and authorisation to:

(i)

complete and execute all or any form(s) of transfer, renunciation or other document(s) in relation to the Celltech Shares referred to in paragraph 1(a)(i) of this Part C in favour of UCB or as UCB or its agents may direct;

(ii)

deliver all or any form(s) of transfer, renunciation and/or other document with any certificate or other document of title for registration within six months of the Offer becoming unconditional in all respects; and

(iii)

take any other action as the agent and/or attorney may think necessary or expedient in connection with his acceptance of the Offer and to vest in UCB (or as it may direct) the Celltech Shares referred to in paragraph 1(a)(i) of this Part C;

(d)

undertakes that the execution of the Form of Acceptance and its delivery to the Receiving Agent, constitutes an irrevocable appointment of the Receiving Agent as his agent and/or attorney and an irrevocable instruction and authority to the agent/attorney:

(i)

subject to the Offer becoming unconditional in all respects and him not having validly withdrawn his acceptance, to transfer to UCB (or to such other person or persons as UCB or its agent may direct) by means of CREST all or any of the Relevant Celltech Shares (as defined below) (but not exceeding the number of Celltech Shares in respect of which the Offer is accepted or deemed to be accepted); and

(ii)

if the Offer does not become unconditional in all respects, to give instructions to CRESTCo immediately after the Offer lapses (or within such longer period as the Panel may permit, not exceeding 14 days of the Offer lapsing) to transfer all Relevant Celltech Shares to the original available balance of the accepting Celltech Shareholder.

45

In this paragraph, "Relevant Celltech Shares" means uncertificated Celltech Shares in respect of which a transfer or transfers to escrow has or have been effected in accordance with the procedures described in the letter from Lazard and paragraph 8 of Part B of this document and where the transfer or transfers to escrow has or have been made in respect of Celltech Shares held under the same member account ID and participant ID as the member account ID and participant ID relating to the relevant Form of Acceptance (but irrespective of whether or not any Form of Acceptance reference number, or a Form of Acceptance reference number corresponding to that appearing on the relevant Form of Acceptance, was included in the relevant transfer to escrow instruction);

(e)

authorises and requests (subject to the Offer becoming unconditional in all respects and him not having validly withdrawn his acceptance):

(i)

Celltech or its agents to procure the registration of the transfer of the Celltech Shares referred to in paragraph 1(a)(i) of this Part C and, in respect of Celltech Shares that are held in certificated form, the delivery of the share certificate(s) and other document(s) of title in respect of the Celltech Shares to UCB or as it may direct;

(ii)

if the Celltech Shares referred to in paragraph 1(a)(i) of this Part C are held in certificated form, to UCB or its agents to procure the despatch by post (or by such other method as may be approved by the Panel) of the consideration to which he is entitled under the Offer at his risk to the person or agent whose name and address is set out in Box 7 of the Form of Acceptance or, if no person or agent's name and address is set out, to the first named holder at his registered address;

(iii)

if the Celltech Shares referred to in paragraph 1(a)(i) of this Part C are held in uncertificated form, to UCB or its agents to ensure that an assured payment obligation is created in favour of the Celltech Shareholder's payment bank in accordance with the CREST assured payment arrangements in respect of any cash consideration to which that shareholder is entitled; and

(iv)

UCB, Celltech or their respective agents to record and act on any instructions with regard to payments or notices which have been entered in the records of Celltech in respect of his holding of Celltech Shares;

(f)

agrees that:

(i)

UCB may decide to despatch all or part of the consideration payable to a Celltech Shareholder whose Celltech Shares are held in uncertificated form in accordance with paragraph 1(e)(ii) of this Part C; and

(ii)

the consideration payable to a shareholder whose Celltech Shares are held in uncertificated form will be despatched in accordance with paragraph 1(e)(iii) of this Part C if the shareholder is a CREST member whose registered address is in a Restricted Jurisdiction;

(g)

gives authority to any director of, or person authorised by, UCB or Lazard within the terms of paragraph 5 of Part B;

(h)

subject to the Offer becoming unconditional in all respects and him not having validly withdrawn his acceptance (or if the Offer will become unconditional in all respects or lapse on the outcome of the resolution in question or if the Panel gives its consent) and pending registration:

(i)

authorises UCB or its agent to direct the exercise of any votes and any other rights and privileges (including the right to requisition the convening of a general or separate class meeting of Celltech) attaching to the Celltech Shares referred to in paragraph 1(a)(i) of this Part C;

(ii)

authorises Celltech or its agent to send any notice, circular, warrant or other document or communication which may be required to be sent to him as a member of Celltech to UCB, care of the Receiving Agent;

(iii)

authorises any director of, or person authorised by, UCB or Lazard to sign any document and do such things as may in the opinion of that agent and/or attorney seem necessary or desirable in connection with the exercise of any votes or other rights or privileges attaching to the Celltech Shares held by him (including, without limitation, signing any consent to short notice of a general or separate class meeting as his agent and/or attorney and on his behalf and executing a form of proxy appointing any person nominated by UCB to attend general and separate class meetings of Celltech and attending any such meeting and exercising the votes attaching to the Celltech Shares

46

referred to in paragraph 1(a)(i) of this Part C on his behalf, where relevant, such votes to be cast so far as possible to satisfy any outstanding condition of the Offer); and

(iv)

agrees not to exercise any such rights without the consent of UCB and irrevocably undertakes not to appoint a proxy for or to attend such general or separate class meetings of Celltech;

This authority will cease to be valid if the acceptance is validly withdrawn in accordance with paragraph 3 of Part B;

(i)

agrees that he will deliver to the Receiving Agent, or procure the delivery to the Receiving Agent of his share certificate(s) and/or other document(s) of title in respect of those Celltech Shares referred to in paragraph 1(a)(i) of this Part C that are held in certificated form, or an indemnity acceptable to UCB, as soon as possible and in any event within two months of the Offer becoming unconditional in all respects;

(j)

agrees that he will take (or procure to be taken) the necessary action to transfer all those Celltech Shares referred to in paragraph 1(a)(i) of this Part C that are held in uncertificated form to an escrow balance as soon as possible and in any event so that the transfer to escrow settles within two months of the Offer becoming unconditional in all respects;

(k)

agrees that if for any reason any Celltech Shares in respect of which a transfer to an escrow balance has been effected are converted to certificated form, he will immediately deliver or ensure the immediate delivery of the share certificates or other documents of title in respect of all those Celltech Shares that are converted to the Receiving Agent;

(l)

agrees that the creation of an assured payment obligation in favour of his payment bank in accordance with the CREST assured payment arrangements as referred to in paragraph 1(e)(iii) of this Part C will, to the extent of the obligation so created, discharge fully any obligation of UCB or Lazard to pay to him the cash consideration to which he is entitled under the Offer;

(m)

agrees that he will do everything necessary or expedient in the opinion of UCB or the Receiving Agent to vest in UCB or its nominees or such other persons as it may decide the Celltech Shares referred to in paragraph 1(a)(i) of this Part C and to enable the Receiving Agent to perform its functions as escrow agent for the purposes of the Offer;

(n)

agrees to ratify everything which may be done or effected by any director of, or person authorised by, UCB, Lazard or the Receiving Agent in exercise of any of the powers and/or authorities under Part B or Part C of this Appendix;

(o)

agrees that, if any provision of Part B or Part C of this Appendix will be unenforceable or invalid or will not operate so as to afford UCB, Lazard or the Receiving Agent or any of their respective directors or persons authorised by them, the benefit of the authority expressed to be given in Part B or Part C of this Appendix, he will, with all practicable speed, do everything that may be required or desirable to enable UCB, Lazard and the Receiving Agent and any of their respective directors or persons authorised by them to secure the full benefit of Part B and Part C of this Appendix;

(p)

represents and warrants that he is entitled to sell and transfer the beneficial ownership of the Celltech Shares referred to in paragraph 1(a)(i) of this Part C and that such shares are sold fully paid and free from all liens, charges, equitable interests, encumbrances, rights of pre-emption and any other rights and interests of any nature whatsoever and together with all rights attaching to them on or after 18 May 2004, including voting rights and the right to receive and retain in full all dividends and other distributions (if any) declared, made or paid on or after that date;

(q)

represents and warrants to UCB and Lazard and the Receiving Agent that, if he is electing for the Loan Note Alternative, he is not in the United States or a resident of a Restricted Jurisdiction, nor is he a US person and he is not accepting the Offer and electing for Loan Notes with a view to the offer, sale or delivery, directly or indirectly, of any such Loan Notes in or into the United States or a Restricted Jurisdiction and he will not hold or acquire any Loan Notes for any other person who he has reason to believe is purchasing for the purpose of that offer, sale or delivery;

(r)

agrees that the terms and conditions of the Offer are deemed to be incorporated in, and form part of, the Form of Acceptance;

47

(s)

agrees that, on execution, the Form of Acceptance takes effect as a deed;

(t)

agrees that the execution of the Form of Acceptance constitutes his agreement to the terms of paragraph 7(t) of Part B;

(u)

agrees and acknowledges that he is not a client (as defined in the rules of the Financial Services Authority) of Lazard, Morgan Stanley or JPMorgan in connection with the Offer; and

(v)

if he is a US Holder, certifies that he is not subject to back up withholding tax by completing Internal Revenue Service Form W-9, or a valid substitute or, if the holder is not a US Holder for US federal income tax purposes, agrees, if relevant, to establish an exemption from certain US federal information return reporting and backup withholding requirements by completing Internal Revenue Service Form W-8BEN which is available from the US Internal Revenue Service.

        A reference in this paragraph to a holder of Celltech Shares includes a reference to the person or persons executing the Form of Acceptance and in the event of more than one person executing a Form of Acceptance, the provisions of this paragraph will apply to them jointly and to each of them.

2      Procedures for Acceptance by Celltech ADS Holders

(a)   Letter of Transmittal/Notice of Guaranteed Delivery

        If you are a holder of Celltech ADSs evidenced by Celltech ADRs, you will have also received a Letter of Transmittal and a Notice of Guaranteed Delivery for use in connection with the Offer. This section should be read together with the instructions on the Letter of Transmittal. The instructions printed on the relevant Letter of Transmittal shall be deemed to form part of the terms of the Offer.

(b)   Valid acceptance

        For a holder of Celltech ADSs evidenced by Celltech ADRs to validly accept the Offer, either:

(i)

a properly completed and duly executed Letter of Transmittal (or a manually signed facsimile thereof), together with any required signature guarantees or, in the case of a book-entry transfer, an Agent's Message, and any other documents required by the Letter of Transmittal, must be received by the Tender Agent and either the Celltech ADRs evidencing such Celltech ADSs must be received by the Tender Agent or such Celltech ADRs evidencing such Celltech ADSs must be delivered pursuant to the procedure for book-entry transfer set forth below (and a Book-Entry Confirmation received by the Tender Agent in accordance with such procedures); or

(ii)

such holder must comply with the Guaranteed Delivery Procedures set out in paragraph 2(h) below.

        The Offer in respect of Celltech ADSs evidenced by Celltech ADRs shall be validly accepted by (i) delivery of a Letter of Transmittal, the relevant Celltech ADRs evidencing Celltech ADSs and any other required documents to the Tender Agent by a holder of Celltech ADSs (without any further action by the Tender Agent) subject to the terms and conditions set out in this document and the Letter of Transmittal or (ii) completion of the book-entry transfer procedures described below. The acceptance of the Offer by a holder of Celltech ADSs evidenced by Celltech ADRs pursuant to the procedures described above, subject to the withdrawal rights described below, will be deemed to constitute a binding agreement between such holder of Celltech ADSs and UCB upon the terms and subject to the conditions of the Offer. If a holder of Celltech ADSs validly accepts the Offer in respect of a Celltech ADS, the Celltech Shares represented by such Celltech ADS may not be tendered independently. A Letter of Transmittal and other required documents contained in an envelope postmarked in a Restricted Jurisdiction or otherwise appearing to UCB or its agents to have been sent from a Restricted Jurisdiction may be rejected as invalid. By delivery of a Letter of Transmittal to the Tender Agent in respect of Celltech ADSs, the holder of such Celltech ADSs agrees to: (a) not instruct the Depositary for the Celltech ADR program to accept the Offer in respect of the Celltech Shares represented by such Celltech ADSs and (b) not deliver such Celltech ADSs to the Tender Agent for the Celltech ADR programme to request withdrawal of the Celltech Shares represented by such Celltech ADSs. UCB may treat as invalid, to the extent that it so determines in its absolute discretion, any acceptance from the Tender Agent for the Celltech ADR programme which UCB has reason to believe has not been properly authorised by the relevant ADS holder and/or is inconsistent with any acceptance received from an ADS holder.

48

(c)   Book-entry transfer

        The Tender Agent will establish an account at the Book-Entry Transfer Facility with respect to Celltech ADSs evidenced by Celltech ADRs held in book-entry form for the purposes of the Offer within two US business days from the date of this document. Any financial institution that is a participant in the Book-Entry Transfer Facility's systems may make book-entry delivery of Celltech ADSs by causing the Book-Entry Transfer Facility to transfer such Celltech ADSs into the Tender Agent's account at such Book-Entry Transfer Facility in accordance with that Book-Entry Transfer Facility's procedures for such transfer.

        Although delivery of Celltech ADSs evidenced by Celltech ADRs may be effected through book-entry transfer into the Tender Agent's account at a Book-Entry Transfer Facility, either:

(i)

the Letter of Transmittal, properly completed and duly executed, together with any required signature guarantees; or

(ii)

an Agent's Message,

and, in either case, any other required documents, must in any case be transmitted to, and received by, the Tender Agent at the relevant address set forth in the Letter of Transmittal before Celltech ADSs evidenced by Celltech ADRs will be either counted as a valid acceptance, or purchased, or such holder must comply with the Guaranteed Delivery Procedures described below. Delivery of documents to a Book-Entry Transfer Facility does not constitute delivery to the Depository.

(d)   Method of delivery

        The method of delivery of Celltech ADRs, Letters of Transmittal and all other required documents is at the option and risk of the accepting holder of Celltech ADSs. Celltech ADSs will be deemed delivered only when the Celltech ADRs evidencing such Celltech ADSs are actually received by the depository (in the case of a book-entry transfer, by Book-Entry Confirmation). If delivery is by mail, registered mail with return receipt requested, properly insured, is recommended. In all cases, sufficient time should be allowed to ensure timely delivery. No acknowledgement of receipt of documents will be given by, or on behalf of UCB.

(e)   Signature guarantees

        No signature guarantee is required on the Letter of Transmittal if:

(i)

the Letter of Transmittal is signed by the registered holder of the Celltech ADSs evidenced by Celltech ADRs in respect of which the Offer is being accepted therewith and such registered holder has not completed either the Box entitled "Special Delivery Instructions" or the Box entitled "Special Payment Instructions" in the Letter of Transmittal; or

(ii)

the Offer is being accepted in respect of such Celltech ADSs for the account of an Eligible Institution.

        In all other cases, all signatures on the Letter of Transmittal must be guaranteed by an Eligible Institution. See Instructions 1 and 5 to the Letter of Transmittal.

(f)    Celltech ADSs and ADRs

        If the Letter of Transmittal is signed by a person other than the registered holder(s) of Celltech ADSs evidenced by Celltech ADRs in respect of which the Offer is being accepted, then such Celltech ADRs must be endorsed or accompanied by appropriate stock powers signed exactly as the name or names of the registered owner or owners appear on the Celltech ADRs. Signatures on such Celltech ADRs or stock powers must be guaranteed by an Eligible Institution. See Instruction 5 to the Letter of Transmittal.

(g)   Partial acceptances (not applicable to book-entry transfers)

        If fewer than all of the Celltech ADSs evidenced by any Celltech ADRs delivered to the Tender Agent are to be tendered, the holder thereof should so indicate in the Letter of Transmittal by filling in the number of Celltech ADSs that are tendered in the Box entitled "Number of ADSs Tendered". In such case, a new Celltech ADR for the untendered Celltech ADSs will be sent to the registered holder, unless otherwise provided in the Letter of Transmittal in the box entitled "Special Delivery Instructions", as promptly as practicable following the date the tendered Celltech ADSs are accepted for payment. All Celltech ADSs delivered to the Tender Agent will be deemed to have been tendered unless otherwise indicated. See Instruction 4 to the Letter of Transmittal.

49

(h)   Guaranteed delivery procedures

(i)

If a holder of Celltech ADSs evidenced by Celltech ADRs wishes to tender Celltech ADSs pursuant to the Offer and the Celltech ADRs evidencing such Celltech ADSs are not immediately available or the procedures for book-entry transfer cannot be completed on a timely basis, or if time will not permit all required documents to reach the Tender Agent prior to the expiration of the Offer, such holder's tender of Celltech ADSs may be effected if all of the following conditions are satisfied (the "Guaranteed Delivery Procedures"):

(a)

such tender is made by or through an Eligible Institution;

(b)

a properly completed and duly executed Notice of Guaranteed Delivery substantially in the form provided by UCB is received by the Tender Agent, as provided below, prior to the expiration of the Offer; and

(c)

the Celltech ADRs evidencing the tendered Celltech ADSs (or, in the case of Celltech ADSs held in book-entry form, timely confirmation of the book-entry transfer of such Celltech ADSs into the Tender Agent's account at the Book-Entry Transfer Facility as described above), together with a properly completed and duly executed Letter of Transmittal (or a manually signed facsimile thereof) with any required signature guarantees (or, in the case of a book-entry transfer, an Agent's Message) and any other documents required by the Letter of Transmittal, are received by the Tender Agent within three New York Stock Exchange business days after the date of execution of such Notice of Guaranteed Delivery.

(ii)

The Notice of Guaranteed Delivery may be delivered by hand, transmitted by facsimile transmission or mailed to the Tender Agent and must include a signature guarantee by an Eligible Institution in the form set out in such Notice of Guaranteed Delivery.

(iii)

Receipt of a Notice of Guaranteed Delivery will not be treated as a valid acceptance for the purpose of satisfying the Acceptance Condition. To be counted towards satisfaction of this requirement, prior to the expiration of the Initial Offer Period, the Celltech ADRs evidencing Celltech ADSs referred to in the Notice of Guaranteed Delivery must be received by the Tender Agent (or, in the case of Celltech ADSs held in book-entry form, timely confirmation of the book-entry transfer of such Celltech ADSs into the Tender Agent's account at the Book-Entry Transfer Facility as described above), together with a properly completed and duly executed Letter of Transmittal (or a facsimile thereof) with any required signature guarantees (or, in the case of a book-entry transfer, an Agent's Message) and any other required documents.

(i)    Other requirements

        By executing the Letter of Transmittal as set out above, the holder of Celltech ADSs evidenced by Celltech ADRs in respect to which the Offer has been accepted will agree that, effective from and after the date all conditions are either satisfied, fulfilled or, to the extent permitted, waived:

(i)

UCB or its agents shall be entitled to direct the exercise of any votes attaching to any Celltech Shares represented by Celltech ADSs, in respect of which the Offer has been accepted or is deemed to have been accepted (the "Accepted ADSs") and any other rights and privileges attaching to such Celltech Shares, including any right to requisition a general meeting of Celltech or any class of its securities;

(ii)

the execution of the Letter of Transmittal (together with any signature guarantees) and its delivery to the Tender Agent or the completion of the book-entry transfer procedures shall constitute:

(a)

an authority to Celltech or its agents from the holder of Accepted ADSs to send any notice, circular, warrant, document or other communication that may be required to be sent to him as a holder of Celltech ADSs to UCB at its registered office;

(b)

an authority to UCB or its agent to sign any consent to short notice of a general meeting or separate class meeting on behalf of the holder of Accepted ADSs and/or to execute a form of proxy in respect of such Accepted ADSs appointing any person nominated by UCB to attend general meetings and separate class meetings of Celltech and any adjournment thereof and to exercise the votes attaching to the Celltech Shares represented by such Accepted ADSs on his behalf;

(c)

the agreement of the holder of Accepted ADSs not to exercise any such rights without the consent of UCB and the irrevocable undertaking of such holder of Accepted ADSs not to appoint a proxy for or to attend any such general meetings or separate class meetings;

50

(d)

a representation and warranty that such holder of Celltech ADSs (i) has not received or sent copies or originals of this document or any Letter of Transmittal or any related documents in, into or from, a Restricted Jurisdiction; (ii) has not used in connection with the Offer or the execution or delivery of the Letter of Transmittal, directly or indirectly, the mails of, or any means or instrumentality (including, without limitation, e-mail, facsimile transmission, telex and telephone) of interstate or foreign commerce of, or any facility of a national securities exchange of a Restricted Jurisdiction; (iii) is accepting the Offer from outside a Restricted Jurisdiction; and (iv) is not an agent or fiduciary acting on a non-discretionary basis for a principal, unless such agent or fiduciary is an authorised employee of such principal or such principal has given any instructions with respect to the Offer from outside a Restricted Jurisdiction;



(e)

confirmation that such holder of Celltech ADSs is entitled to sell and transfer the Accepted ADSs and that such Accepted ADSs are sold fully paid and free from all liens, charges, equitable interests, encumbrances, rights of pre-emption and any other rights and interests of any nature whatsoever and together with all rights now or hereafter attaching thereto, including voting rights and the right to receive all dividends and other distributions (if any) declared, made or paid with respect to the Celltech Shares represented by Celltech ADSs; and



(f)

the execution of the Letter of Transmittal (together with any signature guarantees) and its delivery to the Tender Agent (or the completion of the book-entry transfer procedures) shall constitute an authority in accordance with the terms of paragraph 5 of Part B.

        References in this paragraph to a holder of Celltech ADSs shall include references to the person or persons executing a Letter of Transmittal and, in the event of more than one person executing a Letter of Transmittal, the provisions of this Part C shall apply to them jointly and to each of them.

(j)    Currency of cash consideration

        Holders of Celltech ADSs may elect to receive the cash element of the consideration in pounds sterling. The pounds sterling consideration available to holders of Celltech ADSs is the same, per Celltech Share, as that offered to Celltech Shareholders. To facilitate the settlement of the Offer, unless they elect to receive pounds sterling, holders of Celltech ADSs will receive consideration converted into US dollars at the exchange rate obtainable on the spot market in London on the date the cash consideration is made available by UCB to the Tender Agent for delivery in respect of the relevant Celltech ADSs. A holder of Celltech ADSs may receive such amount on the basis set out above only in respect of the whole of his holding of Celltech ADSs in respect of which he accepts the Offer. Holders of Celltech ADSs may not elect to receive both pounds sterling and US dollars.

        The actual amount of US dollars received will depend upon the exchange rate prevailing on the day on which funds are made available to the Tender Agent by UCB. Holders of Celltech ADSs should be aware that the US dollar/pounds sterling exchange rate which is prevailing at the date on which an election is deemed to be made to receive US dollars and on the dates of despatch and receipt of payment may be different from that prevailing on the day on which funds are made available to the Tender Agent by UCB. In all cases, fluctuations in the US dollar/pounds sterling exchange rate are at the risk of accepting holders of Celltech ADSs who are treated as having elected to receive their consideration in US dollars. None of Celltech, UCB and their advisers or agents shall have any responsibility with respect to the actual amount of cash consideration payable other than in pounds sterling.

3      Substitute Acceptance Forms

        Holders of Celltech Securities have been sent with this document a Form of Acceptance and/or a Letter of Transmittal (accompanied by a Notice of Guaranteed Delivery). All holders of Celltech Shares, including persons in the US who hold Celltech Shares, have been sent a Form of Acceptance, which they must use to tender their Celltech Shares and accept the Offer. All holders of Celltech ADSs have been sent a Letter of Transmittal and a Notice of Guaranteed Delivery which they must use to tender their Celltech ADSs and accept the Offer. Should any holder of Celltech Securities receive an incorrect form with which to accept the Offer or require any additional forms, that person should contact the Helpline.

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APPENDIX II

Particulars of the Loan Notes

        The Loan Notes will be created further to a resolution of the Directors of UCB and will be constituted by the Loan Note Instrument executed as a deed by UCB. The issue of the Loan Notes is conditional, inter alia, on the Offer becoming or being declared unconditional in all respects. The Loan Note Instrument will contain provisions, inter alia, to the following effect:

1      Form and Status

        The Loan Notes will be issued by UCB in amounts and integral multiples of £1 in nominal amount and will constitute unsecured obligations of UCB. The Loan Note Instrument will not contain any restrictions on borrowing, disposals or charging of assets by UCB. All fractional entitlements to the Loan Notes will be disregarded.

2      Interest

(a)

Until such time as the Loan Notes are repaid in full, interest on the Loan Notes will be payable by half yearly instalments in arrears (less any tax required to be deducted) on 31 March and 30 September in each year (an "Interest Payment Date"), except that the first payment of interest will be made on 31 March 2005 (the "First Payment Date"). On the First Payment Date, interest will be paid in respect of the period from (and including) the first date of issue of any of the Loan Notes to (but excluding) the First Payment Date. The period from and including that or any subsequent Interest Payment Date up to but excluding the following Interest Payment Date is an "Interest Period".

(b)

Subject to sub-paragraph (c) below, the rate of interest on the Loan Notes for each Interest Period will be the rate per annum calculated by UCB to be 0.75 per cent. below LIBOR. "LIBOR" means the British Bankers Association Settlement Rate (rounded down, if necessary, to four decimal places), which is quoted as of 11.00 a.m. on the first Business Day of the Interest Period which appears on the Telerate page 3750 at on or about 11.00 a.m. on such day (or such other page or service as may replace it for the purpose of displaying London inter-bank sterling offered rates of leading reference banks) as being the interest rates offered in the London inter-bank market for six-month sterling deposits provided that: if the agreed page is replaced or the service ceases to be available, the rate of interest for each Interest Period shall be 0.75 per cent. below the arithmetic mean (rounded as mentioned above) to be determined instead on the basis of the rate (as quoted to UCB at its request) at which Barclays Bank PLC (or, if that bank is unwilling or unable to provide such a quotation, such other bank in the London inter-bank market as UCB shall reasonably select for the purpose) is offering six month sterling deposits to prime banks in the London inter-bank market at or about 11.00 a.m. on the first Business Day of the relevant Interest Period.

(c)

If a rate of interest cannot be established in accordance with the provisions of sub-paragraph (b) above for any Interest Period, then the rate of interest for that period shall be:

(i)

in the case of the first Interest Period, the rate per annum calculated otherwise in accordance with sub-paragraph (b) but by reference to the first Business Day of that Interest Period on which it can be so established; and

(ii)

in the case of any Interest Period other than the first Interest Period, the same as that applicable to the Loan Notes during the previous Interest Period.

(d)

Payment of interest in respect of the Loan Notes will not be made to addresses in the United States or a Restricted Jurisdiction.

(e)

Each instalment of interest shall be calculated on the basis of a 365-day year and the number of days elapsed in the relevant Interest Period. Interest shall accrue from day to day.

3      Repayment

(a)

A Noteholder may require UCB to repay the whole (whatever the amount) or any part (being £1 nominal or any integral multiple thereof) of the principal amount of his holding of Loan Notes at par, together with accrued interest (subject to any requirement to deduct tax therefrom) up to but excluding the date of repayment, on the First Payment Date and subsequent Interest Payment Dates

52

by lodging a notice of repayment with the Registrar not less than 30 days prior to the relevant redemption date accompanied by the certificate(s) for all the Loan Notes to be repaid.

(b)

If, at any time after 31 March 2005, the principal amount of all Loan Notes outstanding equals or is less than twenty per cent. in total nominal amount of the Loan Notes issued in connection with the Offer, UCB shall be entitled, on giving the remaining Noteholders not less than 30 days' notice in writing to redeem all (but not some only) of the Loan Notes at par together with accrued interest (subject to any requirement to deduct tax therefrom) up to but excluding the date of redemption.

(c)

Any Loan Notes not previously repaid, redeemed or purchased will be repaid in full at par on 30 September 2010, together with accrued interest (subject to any requirement to deduct tax therefrom) up to but excluding that date.

(d)

Each Noteholder will be entitled to require UCB to pay at par all or part (being £1 nominal amount or any integral multiple thereof) of the Loan Notes held by him together with accrued interest (subject to any requirement to deduct tax therefrom) if:

(i)

any principal or interest on any Loan Notes held by that Noteholder is not paid in full by the Company within 30 days of the due date for payment; or

(ii)

the Company or the whole (or substantially the whole) of its assets or undertaking is subject to any form of bankruptcy, liquidation, receivership, administration, arrangement or scheme with creditors, moratorium, interim or provisional supervision by the court or court appointee, whether in the jurisdiction of the place of incorporation or in any other jurisdiction, whether in or out of court (other than a members' voluntary winding-up for the purposes of an amalgamation, reconstruction or merger on terms previously approved by an extraordinary resolution of the noteholders).

(e)

Payment of principal in respect of the Loan Notes will not be made to addresses in the United States or a Restricted Jurisdiction.

4      Purchase of Loan Notes

        UCB may, at any time on or after the date falling six months after the latest date of issue of any outstanding Loan Notes, purchase any Loan Notes at any price by tender (available to all Noteholders alike), by private treaty or otherwise by agreement with the relevant Noteholder(s).

5      Cancellation

        Any Loan Notes repaid, redeemed or purchased will be cancelled and will not be available for re-issue.

6      Registration and transfer

        The Loan Notes will be evidenced by certificates and will be registered. The Loan Notes may be transferred in integral multiples of £1 in nominal value.

        Registered addresses of holders of Loan Notes must be outside the United States and the Restricted Jurisdictions. Documents of title in respect of the Loan Notes will not be sent to addresses in the United States and the Restricted Jurisdictions.

        The Loan Notes may not be offered, sold or delivered, directly or indirectly, in or into the United States or a Restricted Jurisdiction except: (i) pursuant to exemptions from, or in transactions not subject to, the applicable requirements of such jurisdictions; and (ii) with the agreement of the Company.

7      Modifications

        The Noteholders will have power by extraordinary resolution of the Noteholders passed in accordance with the provisions of the Loan Note Instrument or by resolution in writing signed by holders of not less than 75 per cent. of the outstanding Loan Notes, inter alia, to sanction any abrogation, modification or compromise or arrangement in respect of their rights against the Company and to assent to any amendment of the provisions of the Loan Note Instrument. The Company may, with the consent of its financial advisers, amend the provisions of the Loan Note Instrument, without such sanction or consent, if such amendment is of a formal, minor or technical nature or to correct a manifest error.

53

8      Substitution of Principal Debtor

        The Loan Note Instrument will contain, inter alia, provisions enabling UCB to substitute any subsidiary or holding company of UCB as the principal debtor or debtors under the Loan Notes provided that UCB's right to require substitution of such subsidiary or holding company as principal debtor will be exercisable only if UCB guarantees the Loan Notes and if an opinion from leading tax counsel has been obtained (or alternatively in the case of (i) below, the Inland Revenue has confirmed in writing) to the effect that:

(i)

there is no material risk that the substitution will be treated as a disposal of the Loan Notes for the purposes of United Kingdom taxation of chargeable gains;

(ii)

the substitution is either to a company which is a resident for tax purposes in the United Kingdom or will not result in any increase in any withholding tax or deductions in respect of interest payments on the Loan Notes payable to UK resident individuals (as compared with the withholding tax or deductions which would have had to be made by UCB to UK resident individuals who have provided certification as set out in the section entitled "Belgian Withholding Tax" in paragraph 16 of the letter from Lazard set out in Part II of this document); or

(iii)

if the substitution is to a company which is not a resident for tax purposes in the United Kingdom, a UK resident individual will be able to claim a UK tax credit equal to the amount of the deduction or withholding on at least as beneficial a basis as that which applies to any deduction or withholdings which would have had to be made by UCB.

9      Right to Redeem in Euro

        Noteholders may opt to require UCB to pay to him/her in lieu of and in satisfaction of the principal amount of the Loan Notes to be redeemed, an amount of Euro equal to the sterling principal amount of such Loan Notes on the basis set out in the Loan Note Instrument.

10    No Listing

        No application has been made or is intended to be made to any stock exchange for the Loan Notes to be listed or otherwise traded.

11    Governing Law

        The Loan Notes and the Loan Note Instrument will be governed by and construed in accordance with English law.

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APPENDIX III

Financial information relating to Celltech

        The financial information set out in this Appendix III for the three years ended 31 December 2003 has been extracted without material adjustment from the published audited financial statements of Celltech for the three financial years ended 31 December 2001, 2002 and 2003.

        The financial information in this document concerning Celltech does not constitute statutory accounts within the meaning of section 240 of the Companies Act. Copies of the statutory accounts for each of the financial years ended 31 December 2001, 2002 and 2003 have been delivered to the Registrar for Companies for England and Wales pursuant to section 242 of the Companies Act.

        The accounts of Celltech in respect of each of the financial years ended 31 December 2001, 2002 and 2003 were audited by KPMG Audit Plc, Chartered Accountants and Registered Auditors, of 8 Salisbury Square, London EC4Y 8BB. The auditors' report on each of the accounts was unqualified within the meaning of section 235 of the Companies Act and did not contain a statement under section 237(2) or (3) of the Companies Act.

55

CONSOLIDATED PROFIT AND LOSS ACCOUNT

for the year ended 31 December 2003

        The results presented above arise from continuing operations. Oxford GlycoSciences (OGS) has been consolidated as from 14 April 2003. Included in the operating result within the investment in research and development charge of £106.1 million is £3.9 million of costs in respect of continuing projects acquired with OGS. No turnover has been consolidated in respect of OGS.

56

CONSOLIDATED STATEMENT OF TOTAL RECOGNISED GAINS AND LOSSES

for the year ended 31 December 2003

RECONCILIATION OF MOVEMENTS IN SHAREHOLDERS' FUNDS

for the year ended 31 December 2003

57

CONSOLIDATED BALANCE SHEET

as at 31 December 2003

An analysis of shareholders' funds between equity and non-equity interests is given in note 24.

58

CONSOLIDATED CASH FLOW STATEMENT

for the year ended 31 December 2003

*

The total cost of the OGS acquisition including transaction costs was £106.1 million. OGS cash and liquid resources inherited with the acquisition were £126.6 million of which £27.1 million was cash. This results in the net £79.0 million cash outflow reported above (£106.1 million less £27.1 million). The impact of the OGS acquisition on Group cash flows is set out in more detail in note 22.

59

RECONCILIATION OF NET CASH FLOW TO MOVEMENT IN NET FUNDS

for the year ended 31 December 2003