FIRST AMENDMENT TO
                   AGREEMENT AND PLAN OF MERGER
                   ----------------------------

	THIS FIRST AMENDMENT TO AGREEMENT AND PLAN OF
MERGER is dated as of August 31, 2000 (the "Amendment")
among Cistron Biotechnology, Inc., a Delaware corporation
(the "Company"), Celltech Group plc, a company
incorporated under the laws of England ("Parent"), and
CGP Acquisition Corp., a Delaware corporation and wholly-
owned subsidiary of Parent ("Merger Subsidiary").

                        RECITALS

	WHEREAS, the Company, Parent and Merger
Subsidiary entered into an Agreement and Plan of Merger
dated as of March 21, 2000 (the "Original Merger
Agreement" and, as amended by this Amendment, the
"Agreement"), to provide for the merger of Merger
Subsidiary with and into the Company on the terms and
conditions provided therein; and

	WHEREAS, the Company, Parent and Merger
Subsidiary desire to amend the Original Merger Agreement
as hereinafter set forth.

	NOW, THEREFORE, in consideration of the premises
and for other good and valuable consideration, the
parties hereto agree as follows:

	Capitalized terms used herein but not otherwise
defined herein shall have the respective meanings given
such terms in the Agreement.

        1.1.    Section 3.1(a).

        (a)     Section 3.1(a)(1) is hereby amended by
inserting the word "and" at the end thereof.

        (b)     Section 3.1(a)(2) is hereby amended by
deleting at the end thereof "; and" inserting in lieu
thereof:
                .  The Base Merger Consideration and
                Holdback Merger Consideration are
                collectively referred to as the Merger
                Consideration.

        (c)     Section 3.1(a)(3) is hereby deleted in its
entirety.

        (d)     Section 3.1(a) is further amended by
deleting from the last paragraph thereof the words "or
the Aventis Pasteur Option Exercise Merger Consideration".

        1.2.    Section 3.2(b).

        (a)     The third-to-last sentence of Section
3.2(b) is hereby amended by deleting the words "or the
amount of the Aventis Pasteur Option Payments used to
calculate the Aventis Pasteur Option Exercise Merger
Consideration".

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        (b)     The second-to-last sentence of Section
3.2(b) is hereby deleted in its entirety.

        1.3.    Section 3.3.

        (a)     Section 3.3(a) is hereby deleted in its
entirety.

        (b)     Section 3.3(b) is hereby amended to read in
its entirety as follows:
        The Company represents and warrants to Parent and
        Merger Subsidiary that Bluestone Capital Partners, L.P.
        ("Bluestone") has waived any right is has or may have
        to receive any portion of the amounts received by the
        Company, the Surviving Corporation or Parent from
        Aventis Pasteur S.A. ("Aventis Pasteur").  In the event
        Bluestone demands the payment of any such amounts, Parent
        shall have the right, at its discretion, to reduce
        the $1,000,000 amount used to calculate the Aggregate
        Holdback Merger Shares by the amount demanded
        by BlueStone; provided, that to the extent BlueStone
        withdraws such demand or it is finally determined that all
        or part of any such amount demanded by BlueStone is not
        required to be paid to it, no such reduction of the
        $1,000,000 amount used to calculate the Aggregate Holdback
        Merger Shares shall be made.

        (c)     Section 3.3(c) is hereby amended by (i) deleting from
the first sentence thereof the words "or the amount of the Aventis
Pasteur Option Payments used to calculate the Aventis Pasteur Option
Exercise Merger Consideration," and (ii) by deleting the second
sentence thereof in its entirety.

        1.4.    Section 3.4.

        (a)     Section 3.4(a) is hereby amended by (i) deleting the
fourth sentence thereof in its entirety, and (ii) deleting from the
fifth sentence thereof the words "plus the cash portion of the Aventis
Pasteur Option Exercise Merger Consideration."

        (b)     Section 3.4(d) is hereby amended by deleting therefrom
the words "and the Aventis Pasteur Option Exercise Merger Consideration
in accordance with Section 3.3.(a)" and the words "or cash or ADSs (as
applicable) in the case of the Aventis Pasteur Option Exercise Merger
Consideration."

        1.5.    Section 3.5.  Section 3.5 is hereby deleted
in its entirety.

        1.6.    Section 3.7.

        (a)     Section 3.7(c)(ii) is hereby amended by
deleting therefrom subparagraph (i) in its entirety.

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        (b)     Section 3.7(f) is hereby amended by deleting therefrom
the words "or the amount of the Aventis Pasteur Option Payments used to
calculate the Aventis Pasteur Option Exercise Merger Consideration."

        1.7.    Section 6.15.  Section 6.15 is hereby deleted in its entirety.

        1.8.    Section 8.1(d).  Section 8.1(d) is hereby amended by
inserting therein after the word "filed" the words "or submitted in draft
form for confidential review."

        1.9.    Section 9.6.  Section 9.6 is hereby amended by deleting the
Company's notice address and inserting in lieu thereof the following:

        If to the Company:
           By mail:     Cistron Biotechnology, Inc.
                        P.O. Box 2004
                        Pine Brook, New Jersey  07058

	By hand or courier:	Cistron Biotechnology, Inc.
				New Road
                                Building B, Suite 202
                                Parsippany, New Jersey  07054
        1.10.   Section 9.12.

        (a)     The cross reference for the defined term "Aventis Pasteur"
shall be amended to be Section 3.3(b).

        (b)     The following defined terms are hereby deleted:

                Aggregate Aventis Pasteur Option Exercise Merger Shares
                Aggregate Cash Aventis Pasteur Option Exercise Merger
                Consideration
                Aventis Pasteur Deduction
                Aventis Pasteur Option Agreement
                Aventis Pasteur Option Exercise Merger Consideration
                Aventis Pasteur Option Payment

        1.11.   Full Credit for Certain Aventis Pasteur Payments. The parties
acknowledge and agree that, notwithstanding anything in Section 3.2 of the
Company Disclosure Schedule to the contrary, for purposes of calculating
Unaudited Closing Net Current Assets under Section 3.2(b), Cistron will
receive full credit for payments actually received by Cistron from Aventis
Pasteur on or about July 1, 2000 and October 1, 2000 and for the payment
scheduled to be made by Aventis Pasteur to Cistron on January 2, 2001.

        1.12.   Receipt of July 1, 2000 Research Support Payment.  Cistron
represents and warrants to Parent and Merger Subsidiary that Cistron has
received the $75,000 research support payment from Aventis Pasteur due on
July 1, 2000.

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	IN WITNESS WHEREOF, this Amendment has been duly executed and
delivered by the duly authorized representatives of the parties hereto as
of the date first written above.

                                CISTRON BIOTECHNOLOGY, INC.



                                By:/s/ISIDORE S. EDELMAN
                                   ---------------------
                                      Isidore Edelman
                                      Chairman and Chief Executive Officer


                                CELLTECH GROUP PLC


                                By:/s/PETER V. ALLEN
                                   -----------------
                                      Peter V. Allen
                                      Director


                                CGP ACQUISITION CORP.


                                By:/s/PETER V. ALLEN
                                   -----------------
                                      Peter V. Allen
                                      President