Document:

<PAGE>

CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY BRACKETS,
HAS BEEN OMITTED AND FILED SEPARATELY WITH THE SECURITIES AND EXCHANGE
COMMISSION PURSUANT TO RULE 406 OF THE SECURITIES ACT OF 1933, AS AMENDED.

                                                                   EXHIBIT 10.15

                   FIRST AMENDMENT TO COLLABORATION AGREEMENT

This First Amendment to the Collaboration Agreement (this "Amendment") is
entered into as of October __, 1999 by and between EXELIXIS PHARMACEUTICALS,
Inc., a Delaware corporation having its principal place of business at 260
Littlefield Avenue, South San Francisco, California, USA 94080 ("Exelixis"), and
PHARMACIA & UPJOHN AB, a corporation organized and existing under the laws of
Sweden having a place of business at Lindhagensgatan 133, S-112 87 Stockholm,
Sweden ("P&U").

                                    Recitals

     A.  Exelixis and P&U have previously entered into the Collaboration
Agreement dated February 26, 1999 (the "Agreement").  All capitalized terms used
but not otherwise defined herein shall have the meanings given such terms in the
Agreement.

     B.  In accordance with Section 13.5 of the Agreement, Exelixis and P&U
desire to amend the Agreement to expand the Collaboration to include mode of
action projects in the Fields and to cover certain investigational materials
provided by P&U to Exelixis for use in the Collaboration.

     NOW, THEREFORE, Exelixis and P&U agree that the Agreement shall be amended
as provided below:

1.   The following defined terms shall replace the corresponding, current
     defined terms in Article 1 of the Agreement (DEFINITIONS) or, as the case
     may be, be inserted as new defined terms in such Article 1:

     "Candidate Target" [ * ]

     ""Collaboration Compound" means any molecule, other than a P&U Compound,
     that (a) has a molecular weight less than or equal to [ * ], (b) has the
     ability to inhibit, activate or otherwise modulate the activity of a
     Mammalian Target or its encoded protein and (c) is discovered, identified
     or synthesized by or on behalf of P&U or its Affiliate or sublicensee.

     "Exclusive Selected Target" means any Candidate Target, other than a
     Restricted Target, that has been selected as set forth in Section 4.1 of
     the Agreement.

     "Invertebrate Target" is any Target from an invertebrate organism.

                                       1.
<PAGE>

     "Investigational Materials" means (i) tangible samples of drugs, chemicals,
     biologicals and the like ("Basic Materials"), (ii) any P&U Compound, (iii)
     unmodified descendants from Basic Materials, such as virus from virus, cell
     from cell, or organism from organism ("Progeny"), (iv) substances isolated
     from Basic Materials or Progeny which constitute an unmodified functional
     subunit thereof, (v) products expressed by Basic Materials or Progeny
     (e.g., proteins expressed by DNA/RNA, monoclonal antibodies secreted by a
     hybridoma cell line, antibiotic substances elicited from organisms, and the
     like), (vi) substances created by Exelixis which contain/incorporate Basic
     Materials or Progeny or functional subunits thereof or (vii)
     substances/chemical entities created by altering any of the foregoing.

     "Known Target" means a Mammalian Target which is, at the time P&U provides
     the applicable P&U Compound to Exelixis, known to be or believed, based on
     reasonable scientific evidence, to be the target for activity of such P&U
     Compound.

     "Mammalian Target" [ * ]

     "Mode of Action Project(s)" has the meaning assigned to it in Section 3.11.

     "Non-Exclusive Selected Target" means any Restricted Target that has been
     selected as set forth in Section 4.1 of the Agreement.

     "Novel Target" means any Mammalian Target that is not a Known Target.

     "P&U Compound" means a molecule that P&U reasonably believes has
     therapeutic potential in the Field of Metabolic Syndrome or in the Field of
     Alzheimer's Disease and that is provided by P&U to Exelixis for a Mode of
     Action Project in such Field.

     "Product" means any human therapeutic or prophylactic product that
     comprises or incorporates a Collaboration Compound that inhibits, activates
     or otherwise modulates the activity of a Novel Target, but excluding
     products where (i) [ * ] and (ii) [ * ].

     "Research Project" means the planning, execution, and analysis of a
     research project, including without limitation, Mode of Action Projects,
     focused on a particular area of research within a Field based on a mutually
     acceptable definition of a clinical indication, biochemical pathway or
     biological process or related clinical indications, biochemical pathways or
     biological processes.  A Research Project will typically be defined by (a)
     [ * ] and will be initiated with [ * ], or (b) in the case of a Mode of
     Action Project, a P&U Compound.

     "Research Results" means the data and other results generated by Exelixis
     in the course of a Mode of Action Project.

[ * ]= CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY
BRACKETS, HAS BEEN OMITTED AND FILED SEPARATELY WITH THE SECURITIES AND EXCHANGE
COMMISSION PURSUANT TO RULE 406 OF THE SECURITIES ACT OF 1933, AS AMENDED.

                                       2.
<PAGE>

     "Restricted Target" means any Candidate Target for which, on account of
     Exelixis' obligations to a Third Party with respect to such Target: (a)
     Exelixis cannot grant an exclusive license to P&U for such Target and/or
     (b) Exelixis cannot perform any further work within the Collaboration on
     such Target once the identity of such Target becomes known to Exelixis.

     "Royalty-Free Product" means any human therapeutic or prophylactic product
     that (a) comprises or incorporates a Collaboration Compound that inhibits,
     activates or otherwise modulates the activity of a Known Target and (b)
     does not comprise or incorporate a Collaboration Compound that inhibits,
     activates or otherwise modulates the activity of a Novel Target.

     "Selected Target" means an Exclusive Selected Target or a Non-Exclusive
     Selected Target.

     "Target" is any gene or gene product identified in the course of the
     Collaboration or using results generated during the Collaboration,
     including without limitation, an Invertebrate Target, Candidate Target,
     Selected Target, Abandoned Target, Mammalian Target, Novel Target or Known
     Target."

2.   The third sentence in Section 3.1 of the Agreement shall be replaced with
     the following:

     "Each Research Program will involve a number of specific Research Projects,
     each focused on either (a) [ * ] or (b) in the case of a Mode of Action
     Project, a particular P&U Compound for conducting experiments to identify
     genes, proteins and controlling factors involved in a model organism's
     response to such P&U Compound."

3.   A new Section 3.11 shall be inserted in to the Agreement as follows:

     "3.11  Mode of Action Projects.

            (a) The Parties intend to undertake certain "mode of action"
     projects ("Mode of Action Projects") to identify Targets related to the
     action of P&U Compounds for use in discovery and development of small
     molecule drugs to treat humans. Each JSC shall recommend to the JMT and the
     JMT shall determine the number of FTEs to be allocated, [ * ], to the
     performance of Mode of Action Projects in such Research Program. The total
     number of FTEs allocated to the performance of Mode of Action Projects
     under the Collaboration shall not exceed [ * ] without the approval of the
     Parties.

            (b) For each Mode of Action Project, P&U shall provide to Exelixis a
     P&U Compound in a coded, "blind" format without any structural information.
     P&U shall make known to Exelixis the Applicable Field for each P&U Compound
     at the time of delivery of such P&U Compound.

[ * ]= CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY
BRACKETS, HAS BEEN OMITTED AND FILED SEPARATELY WITH THE SECURITIES AND EXCHANGE
COMMISSION PURSUANT TO RULE 406 OF THE SECURITIES ACT OF 1933, AS AMENDED.

                                       3.
<PAGE>

            (c) P&U shall inform Exelixis prior to the start of any Mode of
     Action Project if the P&U Compound investigated in such Mode of Action
     Project has a Mammalian Target that is a Known Target, and P&U shall
     concurrently file documents in escrow with its Legal Department that set
     forth the identity of such Known Target. During the Research Term, Exelixis
     shall not be obliged to work on more than [ * ] which have Mammalian
     Targets that are Known Targets.

            (d) Exelixis shall initially evaluate the feasibility of identifying
     Invertebrate Target(s) for each P&U Compound that it receives. Such initial
     research shall include optimization of the delivery of such P&U Compound to
     a model system organism and analysis of any phenotype arising in said model
     system organism as a result of P&U Compound delivery. Exelixis shall report
     the data arising from such initial research to the JSC.

            (e) The JSC shall review the initial data for each P&U Compound,
     decide whether Exelixis should perform further research on such P&U
     Compound, and prioritize any such further research relative to the other
     work to be performed by Exelixis under the Research Program. Exelixis shall
     proceed in an orderly fashion, based on such prioritization and the number
     of FTEs then committed to the Mode of Action Projects, to perform research
     to identify Invertebrate Target(s) of each such P&U Compound selected by
     the JSC for further work. Such research may include: (i) experiments in
     which [ * ]; (ii) experiments in which [ * ]; and (iii) performance of
     [ *]. Exelixis shall report to the JSC the data arising from such further
     research and the identity of any Invertebrate Target then known by Exelixis
     to be a Restricted Target.

            (f) The JSC shall review all additional data for each P&U Compound,
     select no more than [ * ] Invertebrate Targets (other than Restricted
     Targets) per P&U Compound for molecular analysis by Exelixis, and
     prioritize any such molecular research relative to the other work to be
     performed by Exelixis under the Research Program. Exelixis shall proceed in
     an orderly fashion, based on the JSC's prioritization and the number of
     FTEs then committed to the Mode of Action Projects, to: (i) identify the
     nucleic acid sequence encoding each such Invertebrate Target selected by
     the JSC (unless such sequence is already publicly available); and (ii)
     undertake a good faith search of publicly available databases for the
     mammalian orthologue(s) of such Invertebrate Targets.

            (g) Except as set forth in this Section 3.11(g), the Parties' rights
     and obligations regarding any Target arising from a Mode of Action Project
     shall be identical to that for any Target arising from a Research Project
     other than a Mode of Action Project. In the event that Exelixis discovers
     that a Target identified in a Mode of Action Project is a Restricted
     Target, Exelixis shall immediately cease all work on such Restricted
     Target. Any Restricted Target selected pursuant to Section 4.1 shall be
     deemed a Non-Exclusive Selected Target, and P&U shall have the rights set
     forth in Sections 5.1(b) and 5.1(c) with respect to such Non-Exclusive
     Selected Target. Any Target other than a Restricted Target that is selected
     pursuant to Section 4.1 shall be deemed an Exclusive Selected Target,

[ * ]= CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY
BRACKETS, HAS BEEN OMITTED AND FILED SEPARATELY WITH THE SECURITIES AND EXCHANGE
COMMISSION PURSUANT TO RULE 406 OF THE SECURITIES ACT OF 1933, AS AMENDED.

                                       4.
<PAGE>

     and P&U shall have the rights set forth in Sections 5.1(a) and 5.1(b) with
     respect to such Exclusive Selected Target. The milestone and royalty
     obligations set forth in Sections 7.4 and 7.5 shall apply equally to
     Exclusive Selected Targets and Non-Exclusive Selected Targets and all
     Products arising therefrom."

4.   Section 4.2 of the Agreement shall be replaced with the following:

     "4.2  Pursuit of Selected Targets.  P&U must use good faith Diligent
     Efforts to validate Exclusive Selected Targets or their Mammalian Targets,
     develop assays to assess the activity of Exclusive Selected Targets or
     their Mammalian Targets, use assays to discover Collaboration Compounds
     directed at particular Exclusive Selected Targets or their Mammalian
     Targets, develop and commercialize [ * ] Product per Exclusive Selected
     Target, and pay the applicable royalties set forth in Section 7.5.  P&U's
     diligence obligations under this Section 4.2 for the period prior to the
     initiation of an active research and development program for a
     Collaboration Compound active against a particular Exclusive Selected
     Target or one of its Mammalian Targets will be deemed satisfied if P&U: (i)
     develops a screening assay for the activity of an Exclusive Selected Target
     or one of its Mammalian Targets and initiates screening for modulators of
     the activity of the Exclusive Selected Target or one of its Mammalian
     Targets within [ * ] of the date on which the JSC selected such Exclusive
     Selected Target, provided that, upon reasonable request by P&U, the JMT
     shall grant up to an additional [ * ] and (ii) initiates a program of lead
     optimization and/or medicinal chemistry around lead compounds active in
     such assay within [ * ] of the date on which P&U initiates screening for
     modulators of the activity of such Exclusive Selected Target or one of its
     Mammalian Targets."

5.   Section 4.3 of the Agreement shall be replaced with the following:

     "4.3  Sharing of Biological Data.  P&U shall provide Exelixis with copies
     of all data generated by or on behalf of P&U or its Affiliate or
     sublicensee in the course of validating a Selected Target or a Mammalian
     Target, characterizing the biological function of a Selected Target or a
     Mammalian Target or identifying other genes or proteins that interact with
     a Selected Target or a Mammalian Target.  Exelixis may use such data for
     any purpose other than developing for use in the Applicable Field products
     comprising or incorporating small molecule compounds directed at such
     Selected Target or such Mammalian Target."

6.   Section 4.4(a) of the Agreement shall be replaced with the following:

     "4.4  Target Abandonment.

           (a)  [ * ]"

7.   Section 4.5 of the Agreement shall be replaced with the following:

[ * ]= CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY
BRACKETS, HAS BEEN OMITTED AND FILED SEPARATELY WITH THE SECURITIES AND EXCHANGE
COMMISSION PURSUANT TO RULE 406 OF THE SECURITIES ACT OF 1933, AS AMENDED.

                                       5.
<PAGE>

     "4.5  Targets Other Than Selected Targets.  Exelixis shall retain all
     rights to any Target that (i) does not fulfill the criteria for a Candidate
     Target, (ii) is not selected by the applicable JSC as a Selected Target,
     and such Targets shall not be subject to any terms of this Agreement other
     than those set forth in Section 5.4(b), or (iii) is not a Mammalian
     Target."

8.   Section 4.6 of the Agreement shall be amended by inserting "Mammalian
     Targets," in the first sentence between "Selected Targets," and
     "Collaboration Compounds".

9.   Section 4.7 of the Agreement shall be amended by inserting "Mammalian
     Target" after "Selected Target".

10.  A new Section 4.8 shall be inserted in to the Agreement as follows:

     "4.8  Investigational Materials.

           (a) The transfer of Investigational Materials from P&U to Exelixis is
     essential to the success of the Collaboration. The Investigational
     Materials are and at all times will remain the property of P&U. Nothing in
     this Agreement or the transfer of the Investigational Materials hereunder
     shall be construed to grant an express or implied license to the
     Investigational Materials to Exelixis. Exelixis' rights hereunder shall be
     limited to the right to use the Investigational Materials solely for the
     purposes of this Agreement. P&U warrants that [ * ].

           (b) Exelixis agrees, upon the written request by P&U, to accept those
     Investigational Materials that are P&U Compounds without knowledge of their
     identity or structures, and not to undertake to determine the identity or
     structure of any such Investigational Materials.

           (c) Exelixis agrees that the Investigational Materials are a part of
     P&U's Confidential Information and as such are subject to the obligations
     of confidentiality provided in Article 9 of this Agreement. Exelixis will
     at all times retain control of the Investigational Materials and will
     provide the Investigational Materials only to Exelixis employees who are
     directly involved in providing services under this Agreement. Exelixis
     shall not transfer or disclose to any Third Party any Investigational
     Materials without the prior written consent of P&U.

           (d) P&U shall provide Exelixis with information in P&U's possession
     regarding the safe handling of the Investigational Materials and laws and
     regulations that apply to the use and/or disposal of the Investigational
     Materials, including without limitation those regarding biological
     materials such as NIH or equivalent guidelines for work with recombinant
     DNA.

           (e) THE INVESTIGATIONAL MATERIALS ARE EXPERIMENTAL IN NATURE AND ARE
     PROVIDED WITHOUT ANY WARRANTY AS TO THEIR SAFETY OR FITNESS FOR ANY
     PARTICULAR PURPOSE OR USE.

[ * ]= CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY
BRACKETS, HAS BEEN OMITTED AND FILED SEPARATELY WITH THE SECURITIES AND EXCHANGE
COMMISSION PURSUANT TO RULE 406 OF THE SECURITIES ACT OF 1933, AS AMENDED.

                                       6.
<PAGE>

           (f) Without limiting the warranty provided by P&U in subsection (a)
     above, acceptance of the Investigational Materials will constitute
     Exelixis' acceptance of all liability for any damages or injuries resulting
     from Exelixis' possession or use of the Investigational Materials in a
     manner that (i) does not comply with the safe handling information provided
     by P&U or (ii) is negligent or wrongful.

           (g) Exelixis agrees to return or destroy any unused Investigational
     Materials in accordance with written instructions from P&U."

11.  Section 5.1 of the Agreement shall be replaced with the following:

     "5.1  License to P&U.

           (a) Subject to the terms of this Agreement, Exelixis hereby grants
     P&U an exclusive, worldwide, royalty-bearing license (with the right to
     sublicense) under the Pre-existing Technologies and Sole Inventions
     Controlled by Exelixis and under Exelixis' interest in the Joint Inventions
     (i) to use each Exclusive Selected Target and its Mammalian Target(s) to
     search for Collaboration Compounds directed at such Mammalian Target(s) for
     activity within the Applicable Field, (ii) to develop, for use in the
     Applicable Field, Products and Royalty-Free Products comprising or
     incorporating such Collaboration Compounds, (iii) to develop, following [ *
     ], such Product or Royalty-Free Product for any human indication, and (iv)
     to make, have made, use, sell, offer to sell and have sold such Products
     and Royalty-Free Products.

           (b) Subject to the terms of this Agreement, Exelixis hereby grants
     P&U an exclusive, worldwide, royalty-bearing license (with the right to
     sublicense) to use the Research Results pertaining to Selected Targets in
     the Applicable Field.

           (c) Subject to the terms of this Agreement, Exelixis hereby grants
     P&U a nonexclusive, worldwide, royalty-bearing license (with the right to
     sublicense) under the Pre-existing Technologies and Sole Inventions, in
     each case to the extent Controlled, at the time of Target selection, by
     Exelixis, and under Exelixis' interest in the Joint Inventions (i) to use
     each Non-Exclusive Selected Target and its Mammalian Target(s) to search
     for Collaboration Compounds directed at such Mammalian Target(s) for
     activity within the Applicable Field, (ii) to develop, for use in the
     Applicable Field, Products and Royalty-Free Products comprising or
     incorporating such Collaboration Compounds, (iii) to develop, following [ *
     ], such Product or Royalty-Free Product for any human indication, and (iv)
     to make, have made, use, sell, offer to sell and have sold such Products
     and Royalty-Free Products."

12.  Section 5.2 of the Agreement shall be amended by (i) inserting "Mammalian
     Target," after the first occurrence of "Selected Target," in the first
     sentence, (ii) inserting ", Mammalian Targets" after "Selected Targets" in
     the first sentence, (iii) replacing the

[ * ]= CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY
BRACKETS, HAS BEEN OMITTED AND FILED SEPARATELY WITH THE SECURITIES AND EXCHANGE
COMMISSION PURSUANT TO RULE 406 OF THE SECURITIES ACT OF 1933, AS AMENDED.

                                       7.
<PAGE>

     second occurrence of "Selected
     Target" in the first sentence with "Mammalian Target", and (iv) inserting
     "or Mammalian Target" after "Selected Target in the second sentence.

13.  Section 5.3(a) of the Agreement shall be amended by inserting "and/or its
     Mammalian Target(s)" after each occurrence of "Selected Target".

14.  Section 5.4(a) of the Agreement shall be amended by inserting "or Mammalian
     Target" after each occurrence of "Selected Target" in every sentence but
     the final sentence. In the final sentence, "and its Mammalian Target(s)"
     shall be inserted after the first occurrence of "Selected Target".

15.  The second sentence of Section 5.5 of the Agreement shall be amended by
     replacing "Selected Target" by "Mammalian Target".

16.  The last paragraph of Section 6.2 of the Agreement shall be replaced with
     the following:

     "The exclusivity of the licenses granted to P&U in Sections 5.1(a) and
     5.1(b) shall be subject to the grant of licenses to Third Parties
     consistent with paragraphs (a) and (b) of this Section 6.2.  Upon request
     of the JMT, Exelixis shall consult with the JMT from time to time regarding
     its procedures for seeking to avoid overlapping research activities on
     behalf of multiple Third Parties.  The Parties acknowledge and agree that
     the restrictions set forth in this Section 6.2 shall not apply to any Mode
     of Action Project."

17.  Section 7.4 of the Agreement shall be amended by inserting "[ * ]" in the
     first sentence after "Selected Target,".

18.  Section 7.5 of the Agreement shall be amended by inserting the following
     after the last sentence of the last paragraph thereof:

     "The royalty payments set forth in this Section 7.5 shall apply to every
     Product, regardless of whether it arose from an Exclusive Selected Target
     or a Non-Exclusive Selected Target.  The Parties agree that P&U shall not
     be obliged to make royalty payments to Exelixis for Royalty-Free Products."

19.  The second sentence of Section 9.6 of the Agreement shall be amended by
     inserting ", Mammalian Target" between "Selected Target" and "or Product".

20.  The second sentence of Section 11.3 of the Agreement shall be amended by
     inserting ", Mammalian Targets" between "Selected Targets" and "and
     Collaboration Compounds".

21.  Section 12.1(a) of the Agreement shall be replaced with the following:

     "12.1  Indemnification.

           (a) P&U hereby agrees to defend and hold harmless Exelixis and its
     agents and employees from and against any and all suits, claims, actions,

[ * ]= CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY
BRACKETS, HAS BEEN OMITTED AND FILED SEPARATELY WITH THE SECURITIES AND EXCHANGE
COMMISSION PURSUANT TO RULE 406 OF THE SECURITIES ACT OF 1933, AS AMENDED.

                                       8.
<PAGE>

     demands, liabilities, expenses and/or loss, including reasonable legal
     expenses and reasonable attorneys' fees ("Losses") resulting directly or
     indirectly from (i) the manufacture, use, handling, storage, sale or other
     disposition of chemical agents, Selected Targets, Mammalian Targets,
     Collaboration Compounds or Products by P&U or its Affiliates, agents or
     sublicensees except to the extent such Losses result from the negligence or
     wrongdoing of Exelixis, (ii) Exelixis' infringement of an intellectual
     property right of a Third Party through Exelixis' use of the
     Investigational Materials, or (iii) Exelixis' possession, use or disposal
     of the Investigational Materials in compliance with the safe handling
     information provided by P&U, except to the extent such Losses result from
     the negligence or wrongdoing of Exelixis."

The parties agree that this Amendment shall take effect retroactively as of the
Effective Date of the Agreement (as defined in Section 13.1 of the Agreement).

Except as amended hereby, the Agreement shall remain in full force and effect.

EXELIXIS PHARMACEUTICALS, INC.       PHARMACIA & UPJOHN AB

By: /s/ George A. Scangos, Ph.D.     By: /s/ Goran A. Ando
    ------------------------------       ---------------------------
     Name:   George A. Scangos, Ph.D.      Name:  Goran A. Ando
     Title:  President & CEO               Title: [*] [Executive Vice President,
                                                      Pharmacia & Upjohn, Inc.]

[ * ]= CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY
BRACKETS, HAS BEEN OMITTED AND FILED SEPARATELY WITH THE SECURITIES AND EXCHANGE
COMMISSION PURSUANT TO RULE 406 OF THE SECURITIES ACT OF 1933, AS AMENDED.

                                       9.<PAGE>

                                                                   EXHIBIT 10.16

--------------------------------------------------------------------------------
--------------------------------------------------------------------------------

                            ASSET PURCHASE AGREEMENT

                                     among:

                         Exelixis Pharmaceuticals, Inc.
                            a Delaware corporation;

                                  MetaXen LLC
                     a Delaware limited liability company;

                                      And

                                Xenova Group plc
               a corporation organized under the laws of England.

                               -----------------
                           Dated as of July 11, 1999

                               -----------------

--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
<PAGE>

                               Table Of Contents

<TABLE>
<CAPTION>
                                                                                                                                Page
<S>                                                                                                                             <C>
1.   Purchase and Sale of Assets; Assumption of Liabilities; Related Agreements...............................................   1
     1.1  Assets to be Transferred............................................................................................   1
     1.2  MetaXen Employees...................................................................................................   2
     1.3  Know-How License Grant..............................................................................................   3
     1.4  Right of Negotiation................................................................................................   3
     1.5  Lilly Contract......................................................................................................   3
     1.6  Assignment and Assumption Agreement and Consent between and among
          Purchaser, Seller; Refund of Deposits Under Subleases...............................................................   4
     1.7  Liabilities to be Assumed...........................................................................................   4
     1.8  Liabilities Not to be Assumed.......................................................................................   4
     1.9  Cash Payments and Adjustments at Closing............................................................................   4
     1.10 Further Action......................................................................................................   5
     1.11 Closing.............................................................................................................   5
2.   Representations and Warranties of Seller and Xenova......................................................................   5
     2.1  Due Organization....................................................................................................   5
     2.2  Title to Assets.....................................................................................................   6
     2.3  Contracts...........................................................................................................   6
     2.4  Proceedings; Orders.................................................................................................   7
     2.5  Authority; Binding Nature of Agreements.............................................................................   7
     2.6  Non-Contravention; Consents.........................................................................................   7
     2.7  Brokers.............................................................................................................   8
     2.8  Creditors...........................................................................................................   8
     2.9  Tax Returns and Payments............................................................................................   8
     2.10 Ability.............................................................................................................   8
     2.11 Full Disclosure.....................................................................................................   9
     2.12 Environmental Waste.................................................................................................   9
     2.13 Discrimination Claims...............................................................................................   9
     2.14 Labor Relations.....................................................................................................   9
3.   Representations and Warranties of Xenova.................................................................................   9
</TABLE>

                                      i.
<PAGE>

                               Table Of Contents
                                  (continued)

<TABLE>
<CAPTION>
                                                                                                                                Page
<S>                                                                                                                             <C>
     3.1  Authority of Xenova; Binding Nature of Agreements....................................................................   9
     3.2  Non-Contravention; Consents.........................................................................................  10
     3.3  Ability.............................................................................................................  10
4.   Representations and Warranties of Purchaser..............................................................................  11
     4.1  Due Organization....................................................................................................  11
     4.2  Authority; Binding Nature of Agreement..............................................................................  11
     4.3  Non-Contravention; Consents.........................................................................................  11
5.   Additional Covenants of the Parties......................................................................................  12
     5.1  Further Assurances..................................................................................................  12
6.   Pre-Closing Covenants of the Seller......................................................................................  12
     6.1  Access And Investigation............................................................................................  12
     6.2  Operation Of Business...............................................................................................  13
     6.3  Filings and Consents................................................................................................  14
     6.4  Compliance with Environmental Laws..................................................................................  14
     6.5  Notification; Updates to Disclosure Schedule........................................................................  14
     6.6  No Negotiation......................................................................................................  15
     6.7  Best Efforts........................................................................................................  15
     6.8  Confidentiality.....................................................................................................  15
     7.1  Best Efforts........................................................................................................  15
     7.2  Employment Offers...................................................................................................  15
     7.3  Interference With Seller's Business.................................................................................  15
     7.4  Confidentiality.....................................................................................................  16
     7.5  Filings and Consents................................................................................................  16
8.   Conditions Precedent to the Purchaser's Obligation to Close..............................................................  16
     8.1  Accuracy Of Representations.........................................................................................  16
     8.2  Performance Of Obligations..........................................................................................  16
     8.3  Consents............................................................................................................  17
     8.4  No Material Adverse Change..........................................................................................  17
     8.5  Additional Documents................................................................................................  17
     8.6  No Proceedings......................................................................................................  17
</TABLE>

                                      ii.
<PAGE>

                               Table Of Contents
                                  (continued)

<TABLE>
<CAPTION>
                                                                                                                               Page
<S>                                                                                                                            <C>
     8.7   No Prohibition......................................................................................................  17
9.   Conditions Precedent to the Seller's Obligation to Close.................................................................  17
     9.1   Accuracy Of Representations.........................................................................................  18
     9.2   Purchaser's Performance.............................................................................................  18
10.  Indemnification, Etc.....................................................................................................  18
     10.1  Survival of Representations and Covenants..........................................................................  18
     10.2  Indemnification by Xenova..........................................................................................  19
           (b)    Nonexclusivity of Indemnification Remedies..................................................................  20
     10.3  Indemnification Procedures; Defense of Third Party Claims..........................................................  20
     10.4  Exercise of Remedies by Indemnitees Other Than Purchaser...........................................................  21
     10.5  Indemnification by Purchaser.......................................................................................  21
           (b)    Nonexclusivity of Indemnification Remedies..................................................................  21
     10.6  Indemnification Procedures; Defense of Third Party Claims..........................................................  22
11.  Miscellaneous Provisions.................................................................................................  23
     11.1  Further Assurances.................................................................................................  23
     11.2  Payment of all Federal and State Taxes due on Sale of Assets by Seller.............................................  23
     11.3  Fees and Expenses..................................................................................................  23
     11.4  Attorneys' Fees....................................................................................................  23
     11.5  Notices............................................................................................................  23
     11.6  Publicity..........................................................................................................  25
     11.7  Time of the Essence................................................................................................  25
     11.8  Headings...........................................................................................................  25
     11.9  Counterparts.......................................................................................................  26
     11.10 Governing Law; Dispute Resolution..................................................................................  26
     11.11 Successors and Assigns; Assignment.................................................................................  26
     11.12 Waiver.............................................................................................................  27
</TABLE>

                                     iii.
<PAGE>

                               Table Of contents
                                  (continued)

<TABLE>
<CAPTION>
                                                                                                                              Page
<S>                                                                                                                           <C>
11.13  Amendments............................................................................................................  27
11.14  Severability..........................................................................................................  27
11.15  Parties in Interest...................................................................................................  27
11.16  Entire Agreement......................................................................................................  27
11.17  Construction..........................................................................................................  27
</TABLE>

                                      iv.
<PAGE>

                                    EXHIBITS

Exhibit A:  Employees

Exhibit B:  Certain Definitions

Exhibit C:  Assignment and Assumption Agreement and Consent

Exhibit D:  Form of Opinion (Seller's Counsel)

Exhibit E:  Form of Opinion (Xenova's Counsel)
<PAGE>

                           ASSET PURCHASE AGREEMENT

     This Asset Purchase Agreement is entered into as of July 11, 1999 (the
"Effective Date"), by and between Exelixis Pharmaceuticals, Inc., a Delaware
corporation ("Purchaser"), Xenova Group plc, a corporation organized under the
laws of England ("Xenova") and MetaXen LLC, a Delaware limited liability company
and majority-owned subsidiary of Xenova ("Seller").  Certain capitalized terms
used in this Agreement are defined in Exhibit B.

                                   Recitals

          A.   Seller is engaged and has been engaged in the business of
               providing customized scientific research services in connection
               with the design and synthesis of pharmaceuticals (Seller's
               activities and operations being herein referred to as the
               "Business").

          B.   Seller, together with its parent company, Xenova, have been
               engaged in performing collaborative research pursuant to an
               agreement with Eli Lilly & Co. (the "Lilly Contract" as
               hereinafter defined).

          C.   Seller and Xenova desire to sell and assign to Purchaser certain
               of the assets used in Seller's business, to have Purchaser assume
               certain of Seller's liabilities from such business, and to have
               Purchaser perform certain services on Seller's behalf under the
               Lilly Contract.

          D.   Purchaser desires to purchase such assets and assume such from
               Seller, and perform certain responsibilities on a subcontractor
               basis for Seller under Seller's agreement with Lilly, on the
               terms set forth herein.

                                   Agreement

          Purchaser, Seller and Xenova, intending to be legally bound, agree as
follows:

     1.   Purchase and Sale of Assets; Assumption of Liabilities; Related
     Agreements

          1.1  Assets to be Transferred.  Upon the terms and subject to the
conditions of this Agreement, on the Closing Date (as hereinafter defined),
Seller shall, and Xenova shall cause Seller to, sell, transfer, convey, assign,
grant and deliver to Purchaser, and Purchaser shall purchase, acquire and
receive, the following properties, rights, claims and assets used in the
Business:

               (a) Equipment. All equipment and other assets subject to (i) the
Loan and Security Agreement between MMC/GATX Partnership No.1 and Seller dated
July 31, 1997 (the "MMC Agreement"), which equipment and other assets are
identified in Part 1.1(a)(i) of the Disclosure Schedule, (ii) the Master Lease
Agreement between Comdisco and Seller dated July 24, 1997 (the "Comdisco
Agreement"), which equipment and other assets are identified in Part 1.1(a)(ii)
of the Disclosure Schedule (the "Equipment") and such additional equipment and
other assets owned by Seller as are identified on Part 1.1(a)(iii) of the
Disclosure Schedule.
<PAGE>

               (b) Contracts. All of Seller's rights under the Seller Contracts
listed in Part 1.1(b) of the Disclosure Schedule (the "Assumed Contracts").

               (c) Inventory. All research supplies and such other assets used
in the conduct of Seller's research and development activities (other than the
Equipment) as are listed on Part 1.1(c) of the Disclosure Schedule (the
"Inventory").

               (d) Prepaid Expenses. All of Seller's rights under purchase
orders or contracts for the prepaid items and services listed on Part 1.1(d) of
the Disclosure Schedule (the "Prepaid Expenses").

All of the foregoing assets are hereinafter collectively referred to as the
"Purchased Assets."

All other assets of Seller including, without limitation, proprietary rights of
Seller, Xenova and any third party; accounts receivable; equipment; and other
tangible and intangible personal property, remain the property of Seller and are
not subject to this Agreement.  The parties agree that the removal of such of
assets from the premises described in Section 1.6, including without limitation,
proprietary software, data, databases, and other proprietary or otherwise
confidential information, shall be the responsibility of Seller. Without
limiting the foregoing, certain files, records, data or other assets which are
required for Purchaser to perform its obligations under any subcontract it may
enter into with respect to the Lilly Contract as provided in Section 1.5
("Lilly-Related Materials") shall not be removed from Purchaser's premises until
the end of Purchaser's transitional responsibilities, if any, under any such
subcontract.  During Purchaser's transitional responsibilities under any such
subcontract, Purchaser shall cooperate with Seller to use commercially
reasonable efforts to segregate or otherwise limit access to any Lilly-Related
Materials as necessary to meet any confidentiality and nondisclosure obligations
that Seller may have under or relating to the Lilly Contract.

          1.2  MetaXen Employees.  On or promptly following the Effective Date,
in connection with Purchaser's acquisition of the Purchased Assets pursuant to
this Agreement, Purchaser shall extend employment offers to all Seller employees
set forth on Exhibit A (the "Employees"). Such offers shall be for employment
"at-will" on terms and conditions to be negotiated between Purchaser and such
employees, including without limitation appropriate equity and cash compensation
and benefits packages that when taken as a whole are no less favorable to such
employees than those provided by Seller to such employees. Xenova and Seller
shall use their Best Efforts to avoid the loss of any employees during the
PreClosing Period. If the Closing is delayed until after July 30, 1999 pursuant
to Section 1.11 then Xenova and Seller will use Best Efforts to transfer
Employees not involved in the performance of research and development activities
pursuant to the Lilly Contract to Purchaser during the Pre-Closing Period, and
shall take all actions necessary to enable the transfer of any such Employees
that are performing research and development activities under the Lilly Contract
to Purchaser promptly after the first to occur of (i) the execution of a
subcontract agreement between Seller and Purchaser as set forth in Section 1.5
and the concurrence therein by Lilly, and (ii) the completion of each such
Employee's research and development obligations under the Lilly Contract (with
the transfer of all Employees accepting Purchaser's offer of employment to occur

                                       2
<PAGE>

no later than September 30, 1999). The parties shall cooperate to achieve a
smooth transition for such employees to Purchaser after the Closing Date.

          1.3  Know-How License Grant. The parties acknowledge that in the
course of conducting lead discovery work for Seller, the Employees learned
certain skills that may be covered by Seller's intellectual property rights and
that such Employees are likely to retain such skills. Seller hereby grants to
Purchaser a perpetual, nonexclusive, royalty-free, sublicensable license under
Seller's know-how to continue to practice methods of utilizing equipment,
running tests or performing assays.  Subject to Sections 1.4 and 1.5, such
license shall specifically exclude (i) any rights to practice any patents or to
use any software owned by Seller and (ii) any rights to Seller's proprietary
software, data, databases, or Seller's proprietary equipment, tests or assays
related to ADME, plasminogen activator inhibitor, multidrug resistance, the
performance of the Lilly Contract or other of Seller's and Xenova's proprietary
assays, and (iii) any of Seller's and Xenova's proprietary predictive modeling
technologies.  ("Retained IP")  However, Purchaser may obtain rights under
Retained IP relating to (i) software, (ii) proprietary equipment, tests or
assays related to ADME, and (iii) predictive modeling technologies ("Licensable
IP") by exercising its right of negotiation provided in Section 1.4 and, to the
extent the parties satisfactorily negotiate same, entering into a separate
agreement with Seller governing such a license.

          1.4  Right of Negotiation.  If at any time within one hundred eighty
(180) days after the Closing, Purchaser desires to obtain a nonexclusive license
under Seller's Licensable IP, then it shall so notify Seller in writing.  The
parties shall during the sixty (60) day period following Seller's receipt of
such notice from Purchaser discuss in good faith the terms and conditions under
which Seller shall grant such a license to Purchaser, which terms and conditions
shall be no less favorable than the terms and conditions granted by Seller to
any other third party licensee.

          1.5  Lilly Contract.  As additional consideration for the Purchased
Assets and the proposed employment of the Employees, Purchaser agrees to enter
into an agreement with Seller and Xenova whereunder Purchaser shall agree act as
a subcontractor to Seller to perform Seller's obligations under the research
program being conducted pursuant to the Research and License Agreement between
Seller, Xenova and Eli Lilly and Company ("Lilly") dated February 16, 1998 (the
"Lilly Contract") until October 1, 1999, or such earlier date as the parties and
Lilly may agree. Such agreement shall provide for Purchaser to use all
commercially reasonable efforts to perform Seller's obligations under the Lilly
Contract, including, without limitation, the application of all assets,
equipment and employees of Seller which have previously been applied to such
tasks.  Such agreement shall also provide for (i) Seller to grant to Purchaser
any intellectual property licenses necessary to perform such activities under
the Lilly Contract solely for the purpose of conducting such activities for
Lilly, (ii) Seller to pay to Purchaser a portion of all research support
payments due to Seller pursuant to Section 1.2 from Lilly for Purchaser's
performance of research and development work under the Lilly Contract on a
subcontract basis (with Seller reimbursing Purchaser on an FTE basis for such
work), (iii) Xenova to guarantee Seller's obligations to Purchaser under the
agreement, and (iv) Seller and Xenova to indemnify and hold harmless Purchaser
from and against any claims, liabilities, damages, or suits arising out of or
relating to Purchaser's performance of research and development thereunder,
except to

                                       3
<PAGE>

the extent arising out of Purchaser's gross negligence, recklessness or willful
misconduct. Any delegation of duties and pass through of research support
payments by Seller to Purchaser under the Lilly Contract shall be subject to
Lilly's consent. In the event Lilly's consent thereto cannot be obtained despite
the good faith efforts of the parties, then the parties shall delay the Closing
until Seller completes its obligations to Lilly under the Lilly Contract, which
shall in no event occur after September 30, 1999. Seller and Xenova shall use
Best Efforts to obtain Lilly's consent to the foregoing agreement between Seller
and Purchaser, and if despite using such efforts Lilly does not agree to such
subcontracting arrangement, Seller and Xenova shall use their Best Efforts to
wind down Seller's research and development obligations under the Lilly Contract
as rapidly as practicable in accordance with Seller's obligations under the
Lilly Contract. If Seller, Xenova and Purchaser enter into an agreement as
described in this Section 1.5, Xenova shall retain all rights under the Lilly
Contract, including those with respect to the receipt of milestone payments,
research and development payments for activities performed by Xenova employees
thereunder, and royalties, without any obligation to share such payments or
rights with Purchaser.

          1.6  Assignment and Assumption Agreement and Consent between and among
Purchaser, Seller; Refund of Deposits Under Subleases. At the Closing,
Purchaser, Seller and Xenova shall enter into the Assignment and Assumption
Agreement and Consent in substantially the form attached hereto as Exhibit C
relating to the Build to Suit Lease with Britannia Pointe Grand Limited
Partnership ("Britannia") dated May 27, 1997, as amended (the "Britannia
Agreement"). Purchaser shall assume the Britannia Agreement subject to the
Sublease Agreement between Seller, Britannia and Cytokinetics, Inc. dated May 1,
1998, as amended (the "Cytokinetics Sublease"). Additionally, at the closing,
Seller shall transfer to Purchaser, or credit against amounts due by Purchaser
to Seller pursuant to Section 1.9, the $104,500 security deposit paid by
Cytokinetics, Inc. to Seller pursuant to Section 4(b) of the Cytokinetics
Sublease, and in connection with the foregoing Assignment and Assumption
Agreement and Consent and the Sublease Agreement between Seller, Purchaser and
Britannia dated March 1, 1999, at the Closing Seller shall refund to Purchaser
or credit against amounts due to Seller hereunder the $50,944 security deposit
paid by Purchaser to Seller pursuant to Section 5 of such agreement.

          1.7  Liabilities to be Assumed. Upon the terms and subject to the
conditions of this Agreement, as supplemented by the terms and conditions of the
Assignment and Assumption Agreement and Consent to be entered into pursuant to
Section 1.6, on the Closing Date, Purchaser shall assume and agree to perform
and discharge Seller's Liability arising on and after the Closing Date (i) with
respect to the Purchased Assets and (ii) under and pursuant to the Assumed
Contracts (together, the "Assumed Liabilities").

          1.8  Liabilities Not to be Assumed. Except as, and to the extent
specifically set forth in Section 1.7 with respect to the Assumed Liabilities,
Purchaser is not assuming any other Seller Liabilities (the "Excluded
Liabilities") or Contracts of Seller and all such Excluded Liabilities and
Contracts shall be and remain the responsibility of Seller.

          1.9  Cash Payments and Adjustments at Closing. At the Closing (as
defined in Section 1.11), Purchaser will pay Seller (by check or wire transfer)
an amount equal to the book value of the Equipment, the Inventory and Prepaid
Expenses included in the Purchased

                                       4
<PAGE>

Assets as of the Closing Date, plus any amounts to be reimbursed or credited to
Seller pursuant to Section 1.6 or this Section 1.9, less any amounts to be
reimbursed or credited to Purchaser pursuant to Section 1.6. As of June 30,
1999, the parties have determined that the book value of the Equipment is
$553,570, the book value of the Inventory is $70,000, and the book value of the
Prepaid Expenses is $76,802. In determining amounts due to Seller pursuant to
this Section 1.9, on and as of the Closing Date, the Purchaser and the Seller
shall pro rate all payments made by Seller under the Assumed Contracts in
advance of the Closing Date with respect to payments applicable to the month in
which the Closing Date occurs. In addition, the deposit made by Seller under the
Britannia Agreement shall be remitted to Seller, or in lieu of such remittance,
the amount of such deposits shall be added to the Purchase Price and paid to the
Seller by the Purchaser. On the Closing Date, or as soon as practicable
thereafter, the Seller and Purchaser shall develop in good faith a statement of
adjustments setting forth the various allocations described in this Section 1.9.
Any party owing funds to the other party shall remit such amounts as soon as
practicable, but in any event within 30 days after demand therefor.

          1.10 Further Action. If, at any time after the Closing, any further
action shall be necessary on the part of any party hereto to effect the
intentions of the parties as expressed in this Agreement, each such party shall
take all such further action as may reasonably be necessary to effectuate such
intentions.

          1.11 Closing. The consummation of the transactions contemplated by
this Agreement (the "Closing") shall take place at the offices of Cooley Godward
LLP, 3000 El Camino Real, Palo Alto, California on July 30, 1999 (or at such
other place as Purchaser and Seller shall designate), at 9:00 a.m. (California
time). In the event the Closing is delayed pursuant to Section 1.5, then the
Closing shall occur within five (5) business days following (a) notice from
Seller that it desires to proceed with the Closing or (b) completion of Seller's
research and development obligations under the Lilly Contract; but in any case
not later than September 30, 1999. For purposes of this Agreement, "Closing
Date" shall mean to the time and date as of which the Closing actually takes
place. Each party shall receive such other documents as it may reasonably
request for the purpose of evidencing or effecting the transactions contemplated
by this Agreement, each of which shall be in full force and effect.

2.   Representations and Warranties of Seller and Xenova

     Seller and Xenova jointly and severally represent and warrant as follows:

          2.1  Due Organization.

               (a)  Seller is a limited liability company duly organized under
the laws of the State of Delaware, and has all necessary power and authority:

                    (i)   to conduct its business in the manner in which the
Business is currently being conducted; and

                    (ii)  to own and use its assets in the manner in which its
assets are currently owned and used.

                                       5
<PAGE>

               (b)  Neither Seller nor Xenova has ever approved, or commenced
any proceeding or made any election contemplating, the winding up or cessation
of Seller's business or affairs.

          2.2  Title to Assets. Except as set forth in Part 2.2 of the
Disclosure Schedule, Seller owns, and has good, valid and marketable title to,
all Purchased Assets (it being understood that title to all assets acquired
under the Assumed Contracts which are "leased assets" thereunder shall be
subject only to the terms of such Assumed Contracts. Except as set forth in Part
2.2 of the Disclosure Schedule, all of the Purchased Assets are owned by Seller
free and clear of any Encumbrance.

          2.3  Contracts.

               (a)  Part 1.1(b) of the Disclosure Schedule identifies the
Assumed Contracts. Seller has made available to Purchaser an accurate and
complete copy of the Assumed Contracts identified in Part 1.1(b) of the
Disclosure Schedule, including all amendments thereto, and has disclosed to
Purchaser the amounts owed thereunder.

               (b)  The Assumed Contracts are valid and in full force and
effect, and each is enforceable by the Seller in accordance with its terms,
subject to (i) laws of general application relating to bankruptcy, insolvency
and the relief of debtors and (ii) rules of law governing specific performance,
injunctive relief and other equitable remedies.

               (c)  Except as set forth in Part 2.3(c) of the Disclosure
Schedule:

                    (i)    Seller has not, and to the best of the Knowledge of
Seller, no other Person has, violated or otherwise Breached, or declared or
committed any default under, or termination of, any Assumed Contract;

                    (ii)   To the best of the Knowledge of Seller and Xenova, no
event has occurred, and no circumstance or condition exists, that might (with or
without notice or lapse of time) (A) result in a violation or other Breach of
any of the provisions of the Assumed Contracts, (B) give any Person the right to
declare a default or exercise any remedy under the Assumed Contracts, (C) give
any Person the right to accelerate the maturity or performance of the Assumed
Contracts or (D) give any Person the right to cancel, terminate or modify the
Assumed Contracts;

                    (iii)  Seller has not received any notice or other
communication (in writing or otherwise) regarding any actual, alleged, possible
or potential violation or Breach of, or default under, any of the Assumed
Contracts; and

                    (iv)   Seller has not waived any of its rights under any of
the Assumed Contracts.

               (d)  The performance of the Assumed Contracts will not result in
any violation of or failure to comply with any Legal Requirement.

                                       6
<PAGE>

          2.4  Proceedings; Orders.

               (a)  Except as set forth in Part 2.4(a) of the Disclosure
Schedule, there is no pending Proceeding, and to the best of the Knowledge of
Seller and Xenova, no Person has threatened to commence any Proceeding:

                    (i)    that involves Seller or that otherwise relates to or
might affect the Purchased Assets, the Transactional Agreements or the ability
of Seller or Xenova to perform the Transactions; or

                    (ii)   that challenges, or that may have the effect of
preventing, delaying, making illegal or otherwise interfering with, any of the
Transactions.

Except as set forth in Part 2.4(a) of the Disclosure Schedule, to the best of
the Knowledge of Seller and Xenova, no event has occurred, and no claim, dispute
or other condition or circumstance exists, that might directly or indirectly
give rise to or serve as a basis for the commencement of any such Proceeding.
Seller has delivered to Purchaser accurate and complete copies of all pleadings,
correspondence and other written materials to which Seller has access that
relate to the Proceedings identified in Part 2.4(a) of the Disclosure Schedule.

               (b)  There is no Order to which Seller, or any of the assets
owned or used by Seller, is subject; and Xenova is not subject to any Order that
relates to the Business or to any of the assets owned or used by Seller.

          2.5  Authority; Binding Nature of Agreements.

               (a)  Seller has the right, power and authority to enter into and
to perform its obligations under this Agreement.

               (b)  The execution, delivery and performance by Seller of this
Agreement have been duly authorized by all necessary action on the part of
Seller and its members.

               (c)  This Agreement constitutes the legal, valid and binding
obligation of Seller, enforceable against Seller in accordance with its terms,
subject to (i) laws of general application relating to bankruptcy, insolvency
and the relief of debtors and (ii) rules of law governing specific performance,
injunctive relief and other equitable remedies.

          2.6  Non-Contravention; Consents.  Except as set forth in Part 2.6 of
the Disclosure Schedule, neither the execution and delivery of any of the
Transactional Agreements, nor the consummation or performance of any of the
Transactions, will directly or indirectly (with or without notice or lapse of
time):

               (a)  contravene, conflict with or result in a violation of (i)
any of the provisions of Seller's operating agreement or (ii) any resolution
adopted by Seller's members;

                                       7
<PAGE>

               (b)  contravene, conflict with or result in a violation of, or
give any Governmental Body or other Person the right to challenge any of the
Transactions or to exercise any remedy or obtain any relief under, any Legal
Requirement or any Order to which Seller, or any of the assets owned or used by
Seller, is subject;

               (c)  contravene, conflict with or result in a violation or Breach
of, or result in a default under, any provision of any Assumed Contract;

               (d)  give any Person the right to (i) declare a default or
exercise any remedy under any Seller Contract, (ii) accelerate the maturity or
performance of any Seller Contract or (iii) cancel, terminate or modify any
Assumed Contract; or

               (e)  result in the imposition or creation of any Encumbrance upon
or with respect to the Purchased Assets.

Except as set forth in Part 2.6 of the Disclosure Schedule, Seller was not, is
not or will not be required to make any filing with or give any notice to, or to
obtain any Consent from, any Person in connection with the execution and
delivery of any of the Transactional Agreements or the consummation or
performance of any of the Transactions.

          2.7  Brokers.  Neither Seller nor Xenova has agreed or become
obligated to pay, or has taken any action that might result in any Person
claiming to be entitled to receive, any brokerage commission, finder's fee or
similar commission or fee in connection with any of the Transactions.

          2.8  Creditors.  Part 2.8 of the Disclosure Schedule provides an
accurate and complete list of all creditors of Seller to which Seller owes
$5,000 or more (whether or not such amounts have accrued).  Seller is not in
default of any of its obligations to its creditors.

          2.9  Tax Returns and Payments.  Seller has filed all Tax returns and
reports as required by law.  Seller has paid all Taxes and other assessments due
and has made adequate provision for Taxes due or accrued as of the date hereof.
Seller has not been advised of any Tax deficiency proposed or assessed against
it and has not executed any waiver of any statute of limitations on the
assessment or collection of any tax or governmental charge.  Seller has not been
advised that any of its federal income tax returns or state income or franchise
tax or sales or use tax returns have been audited by governmental authorities.
Seller has withheld or collected from each payment made to each of its employees
the amount of all Taxes required to be withheld or collected therefrom, and has
paid the same to the proper tax receiving officers or authorized depositaries.

          2.10 Ability.  Seller has not, at any time, (A) made a general
assignment for the benefit of creditors, (B) filed or had filed against it any
bankruptcy petition or similar filing, (C) suffered the attachment or other
judicial seizure of all or a substantial portion of Seller's assets, (D)
admitted in writing Seller's inability to pay Seller's debts as they become due,
(E) been convicted of, or pleaded guilty to, any felony or (F) taken or been the
subject of any action that

                                       8
<PAGE>

may have an adverse effect on Seller's ability to comply with or perform any of
its covenants or obligations under any of the Transactional Agreements.

          2.11 Full Disclosure.

               (a)  This Agreement does not (i) contain any representation,
warranty or information of or with respect to Seller or Xenova that is false or
misleading with respect to any material fact, or (ii) omit to state any material
fact necessary in order to make the representations, warranties and information
of or with respect to Seller or Xenova contained and to be contained herein and
therein (in the light of circumstances under which such representations,
warranties and information were or will be made or provided) not false or
misleading.

               (b)  All of the information set forth in the Disclosure Schedule
is accurate and complete in all respects.

          2.12 Environmental Waste. Seller is in compliance in all material
respects with applicable Environmental Laws, including without limitation those
relating to maintenance, use or disposal of hazardous materials.  There is no
Proceeding, notice of violation, or demand letter pending or threatened against
the Seller relating in any way to the Environmental Laws.

          2.13 Discrimination Claims.  There are no discrimination charges
relating to sex, sexual harassment, age, race, national origin, mental or
physical disability, or other protected category, pending, or to the best of
Seller's knowledge, threatened against Seller, or involving Seller or its
employees, before any federal, state, county or local court, agency, board,
commission, authority or other subdivision thereof.

          2.14 Labor Relations.  There is no unfair labor practice complaint, or
any Proceeding under the National Labor Relations Board, pending against the
Seller, or to the best of Seller's knowledge, threatened against the Seller.
There is no labor strike or similar dispute pending or to the best of Seller's
knowledge threatened against or involving Seller.  Seller is not a party to or
bound by any collective bargaining agreement with any employees of Seller, and
no collective bargaining agreement is currently being negotiated by Seller with
respect to its employees.

3.        Representations and Warranties of Xenova

     Xenova represents and warrants as follows:

          3.1  Authority of Xenova; Binding Nature of Agreements.  Xenova has
the right, power and capacity to enter into and to perform Xenova's obligations
under each of the Transactional Agreements to which Xenova is or may become a
party.  This Agreement constitutes the legal, valid and binding obligation of
Xenova, enforceable against Xenova in accordance with its terms, subject to (i)
laws of general application relating to bankruptcy, insolvency and the relief of
debtor, and (ii) rules of law governing specific performance, injunctive relief
and other equitable remedies.  Upon the execution of each of the other
Transactional Agreements at the Closing, each of such other Transactional
Agreements to which

                                       9
<PAGE>

Xenova becomes a party will constitute the legal, valid and binding obligation
of Xenova, and will be enforceable against Xenova in accordance with its terms,
subject to (i) laws of general application relating to bankruptcy, insolvency
and the relief of debtor, and (ii) rules of law governing specific performance,
injunctive relief and other equitable remedies.

          3.2  Non-Contravention; Consents.  Except as set forth in Part 3.2 of
the Disclosure Schedule, neither the execution and delivery of any of the
Transactional Agreements, nor the consummation or performance of any of the
Transactions, will directly or indirectly (with or without notice or lapse of
time):

               (a)  contravene, conflict with or result in a violation of, or
give any Governmental Body or other Person the right to challenge any of the
Transactions or to exercise any remedy or obtain any relief under, any Legal
Requirement or any Order to which Xenova is subject; or

               (b)  contravene, conflict with or result in a violation or Breach
of or a default under any provision of, or give any Person the right to declare
a default under, any Contract to which Xenova is a party or by which Xenova is
bound.

          3.3  Ability.

               (a)  Xenova:

                    (i)   has not, at any time, (A) made a general assignment
for the benefit of creditors, (B) filed, or had filed against Xenova, any
bankruptcy petition or similar filing, (C) suffered the attachment or other
judicial seizure of all or a substantial portion of Xenova's assets, (D)
admitted in writing Xenova's inability to pay Xenova's debts as they become due,
(E) been convicted of, or pleaded guilty to, any felony, or (F) taken or been
the subject of any action that may have an adverse effect on Xenova's ability to
comply with or perform any of Xenova's covenants or obligations under any of the
Transactional Agreements; and

                    (ii)  is not subject to any Order that may have an adverse
effect on Xenova's ability to comply with or perform any of Xenova's covenants
or obligations under any of the Transactional Agreements.

               (b)  There is no Proceeding pending, and, to Xenova's Knowledge,
no Person has threatened to commence any Proceeding, that may have an adverse
effect on the ability of Xenova to comply with or perform any of Xenova's
covenants or obligations under any of the Transactional Agreements. To Xenova's
knowledge, no event has occurred, and no claim, dispute or other condition or
circumstance exists, that might directly or indirectly give rise to or serve as
a basis for the commencement of any such Proceeding.

                                       10
<PAGE>

     4.  Representations and Warranties of Purchaser

     Purchaser represents and warrants as follows:

          4.1 Due Organization. Purchaser is a corporation duly organized,
validly existing and in good standing under the laws of the State of Delaware.

          4.2 Authority; Binding Nature of Agreement.

               (a) Purchaser has the right, power and authority to enter into
and perform its obligations under this Agreement.

               (b) The execution, delivery and performance of this Agreement by
Purchaser have been duly authorized by all necessary action on the part of
Purchaser and its Board of Directors.

               (c) This Agreement constitutes the legal, valid and binding
obligation of Purchaser, enforceable against Purchaser in accordance with its
terms, subject to (i) laws of general application relating to bankruptcy,
insolvency and the relief of debtor, and (ii) rules of law governing specific
performance, injunctive relief and other equitable remedies.

          4.3 Non-Contravention; Consents. Neither the execution and delivery of
any of the Transactional Agreements, nor the consummation or performance of any
of the Transactions, will directly or indirectly (with or without notice or
lapse of time):

              (a) contravene, conflict with or result in a violation of (i) any
of the provisions of Purchaser's Certificate of Incorporation or bylaws or (ii)
any resolution adopted by Purchaser's stockholders or Purchaser's Board of
Directors; or

              (b) contravene, conflict with or result in a violation of, or give
any Governmental Body or other Person the right to challenge any of the
Transactions or to exercise any remedy or obtain any relief under, any Legal
Requirement or any Order to which Purchaser or any of the assets owned or used
by Purchaser is subject.

              (c) contravene, conflict with or result in a violation of, or give
any third party the right to challenge any of the Transactions or to exercise
any remedy under any material agreement of Purchaser.

              (d) Purchaser was not, is not and will not be required to make any
filing with or give any notice to, or to obtain any Consent from, any Person in
connection with the execution and delivery of any of the Transactional
Agreements or the consummation or performance of any of the Transactions.

              (e) Purchaser has the financial wherewithall to assume and perform
the Assumed Liabilities.

                                       11
<PAGE>

     5.  Additional Covenants of the Parties

          5.1 Further Assurances.

              (a) If, at any time after Closing, any further action is
determined by any party to this Agreement to be reasonably necessary or
desirable to carry out the purposes of this Agreement, Purchaser, Seller and
Xenova each agree to take such action.

              (b) To the extent that the Assumed Contracts (other than the MMC
Agreement, the Comdisco Agreement and the Britannia Agreement, the assumption of
which shall be a condition to Closing) for which assignment to Purchaser is
provided pursuant to Section 1.1(a) are not assignable without the Consent of
another Person, this Agreement shall not constitute an assignment or an
attempted assignment thereof if such assignment or attempted assignment would
constitute a breach thereof. Xenova, Seller and Purchaser agree to use
commercially reasonable Best Efforts (without any requirement on the part of
Purchaser to pay any money or agree to any change in the terms of such Contract)
to obtain the Consent of such other Person to the assignment of such Assumed
Contracts to Purchaser in all cases in which such Consent is or may be required
for such assignment. If any such Consent shall not be obtained, Xenova and
Seller agree to cooperate with Purchaser in any reasonable arrangement mutually
satisfactory to all parties designed to provide for Purchaser the benefits
intended to be assigned to Purchaser under the Assumed Contracts, including
enforcement at the cost of Seller and for the account of Purchaser of any and
all rights of Seller against the other party thereto arising out of the breach
or cancellation thereof by such other party or otherwise. If, and to the extent
that, such arrangement cannot be made, Purchaser, upon notice to Seller, shall
have no obligation pursuant to this Agreement with respect to such Assumed
Contracts (other than the MMC Agreement, the Comdisco Agreement and the
Britannia Agreement, the assumption of which shall be a condition to Closing)
and such Assumed Contracts shall not be deemed to be a Purchased Asset. In such
event, the parties shall negotiate in good faith an adjustment to the
consideration paid by Purchaser for the Purchased Assets to reflect the Assumed
Contracts not transferred to Purchaser and the fees, costs and expenses
(including reasonable attorneys' fees) incurred by Purchaser in connection with
the parties' efforts to assign the Assumed Contracts to Purchaser.

     6. Pre-Closing Covenants of the Seller.

          6.1 Access And Investigation. Xenova and the Seller shall ensure that,
at all times during the Pre-Closing Period: (a) the Seller and its
Representatives provide the Purchaser and its Representatives with such
reasonable access to the Seller's Representatives, personnel and assets and to
all existing books, records, Tax Returns, work papers and other documents and
information relating to the Purchased Assets and the Transactions as necessary
to permit Purchaser to complete its reasonable due diligence obligations with
respect thereto; (b) the Seller and its Representatives provide the Purchaser
and its Representatives with such copies of existing books, records, Tax
Returns, work papers and other documents and information relating to the
Purchased Assets and Assumed Liabilities as the Purchaser may reasonably request
in good faith; and (c) the Seller and its Representatives compile and provide
the Purchaser and its

                                       12
<PAGE>

Representatives with such additional financial, operating and other data and
information relating to the Purchased Assets and Assumed Liabilities as the
Purchaser may request in good faith.

          6.2 Operation Of Business. Xenova and the Seller shall ensure that,
during the Pre-Closing Period:

              (a) the Seller conducts its operations relating to the Purchased
Assets or otherwise to the Transactions exclusively in the Ordinary Course of
Business and in the same manner as such operations have been conducted prior to
the date of this Agreement;

              (b) the Seller, to the extent related to the Purchased Assets and
the Transactions, uses commercially reasonable Best Efforts to (i) preserve
intact its current business organization, (ii) keep available the services of
its current officers and employees, (iii) maintain its relations and good will
with all landlords, creditors, licensors, licensees, employees, independent
contractors and other Persons having business relationships with the Seller and
(iv) promptly repair, restore or replace any Purchased Assets that are destroyed
or damaged;

              (c) the officers of the Seller confer regularly with the Purchaser
concerning operational matters and otherwise report regularly to the Purchaser
concerning the status of the Seller's assets, liabilities, and operations to the
extent related to the Purchased Assets and the Transactions;

              (d) the Seller does not take any actions that may increase the
amounts due by Seller under the MMC Agreement or the Comdisco Agreement;

              (e) the Seller does not effect or become a party to any
Acquisition Transaction;

              (f) the Seller does not enter into or permit any of the Purchased
Assets to become bound by any Contract (except the Lilly Contract or as
expressly provided for in Section 1);

              (g) the Seller does not commence or settle any Proceeding relating
to the Purchased Assets or the Transactions;

              (h) the Seller does not enter into any transaction or take any
other action outside the Ordinary Course of Business with respect to the
Purchased Assets or the Transactions;

              (i) the Seller does not enter into any transaction or take any
other action that might cause or constitute a Breach of any representation or
warranty made by Xenova or the Seller in this Agreement;

              (j) Seller will not discourage Employees from accepting
Purchaser's offer of employment or take any action that would interfere with the
Employees' ability or willingness to accept Purchaser's offer of employment; and

                                       13
<PAGE>

              (k) the Seller does not agree, commit or offer (in writing or
otherwise) to take any of the actions described in clauses "(d)" through "(j)"
of this Section 6.2.

          6.3 Filings and Consents. Xenova and Seller shall ensure that: (a) all
filings, notices and Consents required to be made, given and obtained in order
to consummate the Transactions are made, given and obtained on a timely basis;
and (b) during the Pre-Closing Period, Xenova and Seller shall each cooperate
with the other and with each others' Representatives, and prepare and make
available such documents and take such other actions as the others may request
in good faith, in connection with any filing, notice or Consent that such party
is required or elects to make, give or obtain.

          6.4 Compliance with Environmental Laws. Seller shall operate its
Business during the Pre-Closing Period in compliance in all material respects
with all limitations, restrictions, conditions, standards, prohibitions,
requirements, obligations, schedules and timetables provided in the
Environmental Laws, including without limitation those relating to maintenance
or disposal of hazardous materials and shall provide Purchaser with
documentation regarding same. Without limitation, during the Pre-Closing Period,
Seller shall dispose of all hazardous material waste that is located on the
premises to be leased to Purchaser pursuant to the Britannia Agreement and the
Assignment and Assumption and Consent Agreement to be entered into pursuant to
Section 1.6 in accordance with applicable Environmental Laws, at Seller's sole
expense.

          6.5 Notification; Updates to Disclosure Schedule. During the Pre-
Closing Period, Xenova and the Seller shall promptly notify the Purchaser in
writing of: (a) the discovery by Xenova or the Seller of any event, condition,
fact or circumstance that occurred or existed on or prior to the date of this
Agreement and that caused or constitutes a Breach of any representation or
warranty made by Xenova or the Seller in this Agreement; (b) any event,
condition, fact or circumstance that occurs, arises or exists after the date of
this Agreement and that would cause or constitute a Breach of any representation
or warranty made by Xenova or the Seller in this Agreement if (i) such
representation or warranty had been made as of the time of the occurrence,
existence or discovery of such event, condition, fact or circumstance, or (ii)
such event, condition, fact or circumstance had occurred, arisen or existed on
or prior to the date of this Agreement; (c) any Breach of any covenant or
obligation of Xenova or the Seller; and (d) any event, condition, fact or
circumstance that may make the timely satisfaction of any of the conditions set
forth in Section 8 impossible or unlikely. If any event, condition, fact or
circumstance that is required to be disclosed pursuant to this Section 6.5
requires any change in the Disclosure Schedule, or if any such event, condition,
fact or circumstance would require such a change assuming the Disclosure
Schedule were dated as of the date of the occurrence, existence or discovery of
such event, condition, fact or circumstance, then Xenova and the Seller shall
promptly deliver to the Purchaser an update to the Disclosure Schedule
specifying such change. No such update shall be deemed to supplement or amend
the Disclosure Schedule for the purpose of determining whether any of the
conditions set forth in Section 8 has been satisfied. In the event any such
change in the Disclosure Schedule results in a material change in the Seller's
ability to make the representations and warranties made herein, then Purchaser
shall be entitled not to conduct the Closing in view of such update or may
request that Xenova and Seller agree in writing to negotiate with Purchaser in
good faith an appropriate modification to

                                       14
<PAGE>

the Purchase Price to take into account any such supplemental information, which
request shall not be unreasonably denied.

          6.6 No Negotiation. Xenova and the Seller shall ensure that, during
the Pre-Closing Period, neither Xenova nor the Seller, nor any Representatives
of Xenova or Seller, directly or indirectly: (a) solicits or encourages the
initiation of any inquiry, proposal or offer from any Person (other than the
Purchaser) relating to any Acquisition Transaction; (b) participates in any
discussions or negotiations with, or provides any non-public information to, any
Person (other than the Purchaser) relating to any proposed Acquisition
Transaction; or (c) considers the merits of any unsolicited inquiry, proposal or
offer from any Person (other than the Purchaser) relating to any Acquisition
Transaction.

          6.7 Best Efforts. During the Pre-Closing Period, Xenova and Seller
shall use their commercially reasonable Best Efforts to cause the conditions set
forth in Section 8 to be satisfied on a timely basis.

          6.8 Confidentiality. Except as provided in Section 11.6, Xenova and
Seller shall ensure that, during the Pre-Closing Period, neither Seller nor
Xenova, nor any of their Representatives, issues or disseminates any press
release or other publicity or otherwise makes any disclosure of any nature to
any other Person regarding any of the Transactions or the existence or terms of
this Agreement, except to the extent that Xenova or Seller is required by law to
make any such disclosure. Xenova or Seller shall provide to Purchaser a draft of
any disclosure to be made pursuant to this Section 6.8 for review and comment by
Purchaser at least one (1) business day in advance of the date upon which such
release shall be made.

     7. Pre-Closing Covenants of the Purchaser.

          7.1 Best Efforts. During the Pre-Closing Period, the Purchaser shall
use its commercially reasonable Best Efforts to cause the conditions set forth
in Section 9 to be satisfied.

          7.2 Employment Offers. Purchaser shall extend offers of employment to
the individuals listed on Exhibit A on the terms and conditions set forth in
Section 1.2 hereof, such employment to become effective as of the Closing.

          7.3 Interference With Seller's Business. Purchaser shall not take any
action between signing and closing to interfere with Seller's conduct of its
business in the ordinary course. Without limiting the foregoing, during the Pre-
Closing Period Purchaser shall not extend offers of employment to any
individuals listed on Exhibit A other than on the terms and conditions set forth
in Section 1.2 hereof and only in connection with the occurrence of the Closing
hereunder. Should such Closing not occur due to the material breach of this
Agreement by Purchaser, then Purchaser shall withdraw any such employment offers
and shall not further pursue the employment of any Seller employee except as
consented in writing by Seller. Should such Closing not occur for any other
reason (other than material breach by Seller or Xenova), then Purchaser shall
not extend any employment offers to or take any other action to employ the
individuals listed on Exhibit A then employed by Seller and involved in research
and

                                       15
<PAGE>

development under the Lilly Contract for employment by Purchaser to commence
prior to the earlier to occur of (i) Seller's completion of its research and
development obligations under the Lilly Contract, and (ii) September 30, 1999.

          7.4 Confidentiality. Except as provided in Section 11.6, Purchaser
shall ensure that, during the Pre-Closing Period: neither it, nor any of its
Representatives, issues or disseminates any press release or other publicity or
otherwise makes any disclosure of any nature to any other Person regarding any
of the Transactions or the existence or terms of this Agreement, except to the
extent that Purchaser is required by law to make any such disclosure. Purchaser
shall provide to the other parties a draft of any disclosure to be made pursuant
to this Section 7.4 for review and comment by the other parties at least one (1)
business day in advance of the date upon which such release shall be made.

          7.5 Filings and Consents. Purchaser shall ensure that: (a) all
filings, notices and Consents required to be made, given and obtained in order
to consummate the Transactions are made, given and obtained on a timely basis;
and (b) during the Pre-Closing Period, Purchaser shall cooperate with the other
and with its Representatives, and prepare and make available such documents and
take such other actions as the others may request in good faith, in connection
with any filing, notice or Consent that Purchaser is required or elects to make,
give or obtain.

     8. Conditions Precedent to the Purchaser's Obligation to Close.

     The Purchaser's obligation to purchase the Assets and to take the other
actions required to be taken by the Purchaser at the Closing is subject to the
satisfaction, at or prior to the Closing, of each of the following conditions
(any of which may be waived by the Purchaser, in whole or in part, in writing):

          8.1 Accuracy Of Representations. All of the representations and
warranties made by Xenova and the Seller in this Agreement (considered
collectively), and each of said representations and warranties (considered
individually), shall have been accurate in all material respects as of the date
of this Agreement, and shall be accurate in all material respects as of the
Closing Date as if made at the Closing Date, without giving effect to any update
to the Disclosure Schedule.

          8.2 Performance Of Obligations.

              (a) The Assignment and Assumption Agreement and Consent described
in Section 1.6 relating to the Britannia Agreement shall have been executed by
each of the parties thereto and delivered to the Purchaser.

              (b) Assignment and Assumption Agreements with respect to the
MMC/GATX Loan and Security Agreement and the Comdisco Master Lease Agreement,
each as described in Section 1.1(a), shall have been executed by the Purchaser
and the respective lenders.

                                       16
<PAGE>

              (c) All of the covenants and obligations that Xenova and the
Seller are required to comply with or to perform at or prior to the Closing
(considered collectively), and each of said covenants and obligations
(considered individually), shall have been duly complied with and performed in
all material respects.

          8.3 Consents. Each of the Consents identified in Part 8.3 of the
Disclosure Schedule shall have been obtained and shall be in full force and
effect.

          8.4 No Material Adverse Change. There shall have been no material
adverse change in the business, condition, assets, liabilities, operations,
financial performance, net income or prospects of the Seller relating to the
Purchased Assets or the Transactions since the date of this Agreement, and no
event shall have occurred and no condition or circumstance shall exist that
could be expected to give rise to any such material adverse change.

          8.5 Additional Documents. Purchaser shall have received the following
documents:

              (a) an opinion letter from Wilson, Sonsini, Goodrich & Rosati,
counsel to the Seller, dated the Closing Date, in the form of Exhibit D, and an
opinion letter from Xenova's counsel, dated the Closing Date, in the form of
Exhibit E; and

              (b) such other documents as the Purchaser may request in good
faith for the purpose of (i) evidencing the accuracy of any representation or
warranty made by Xenova or the Seller, (ii) evidencing the compliance by Xenova
or the Seller with, or the performance by Xenova or the Seller of, any covenant
or obligation set forth in this Agreement, (iii) evidencing the satisfaction of
any condition set forth in this Article 8 or (iv) otherwise facilitating the
consummation or performance of any of the Transactions.

          8.6 No Proceedings. Since the date of this Agreement, there shall not
have been commenced or threatened against the Purchaser, or against any Person
affiliated with the Purchaser, any Proceeding (a) involving any material
challenge to, or seeking material damages or other material relief in connection
with, any of the Transactions, or (b) that may have the effect of preventing,
delaying, making illegal or otherwise interfering with any of the Transactions.

          8.7 No Prohibition. Neither the consummation nor the performance of
any the Transactions will, directly or indirectly (with or without notice or
lapse of time), contravene or conflict with or result in a violation of, or
cause the Purchaser or any Person affiliated with the Purchaser to suffer any
adverse consequence under, any applicable Legal Requirement or Order.

     9. Conditions Precedent to the Seller's Obligation to Close.

     The Seller's obligation to sell the Purchased Assets and to take the other
actions required to be taken by the Seller and Xenova at the Closing is subject
to the satisfaction, at or prior to the Closing, of each of the following
conditions (any of which may be waived by Xenova's Representative, in whole or
in part, in writing):

                                       17
<PAGE>

          9.1 Accuracy Of Representations. All of the representations and
warranties made by the Purchaser in this Agreement (considered collectively),
and each of said representations and warranties (considered individually), shall
have been accurate in all material respects as of the date of this Agreement and
shall be accurate in all material respects as of the Closing Date as if made at
the Closing Date.

          9.2 Purchaser's Performance.

              (a) The Purchaser shall have executed and delivered (i) the
assignment and Assumption Agreement and Consent described in Section 1.6; (ii)
the Assignment and Assumption Agreement with Comdisco; (iii) the Assignment and
Assumption Agreement with MMC/GATX; and (iv) the employment offer letters on the
terms and conditions set forth in Section 1.2 to the Employees; and shall have
made the payment and assume the obligations contemplated by Section 1.9.

              (b) All of the other covenants and obligations that the Purchaser
is required to comply with or to perform pursuant to this Agreement at or prior
to the Closing (considered collectively), and each of said covenants and
obligations (considered individually), shall have been complied with and
performed in all material respects.

     10. Indemnification, Etc.

          10.1 Survival of Representations and Covenants.

              (a) The representations and warranties made by Seller and Xenova
in this Agreement (including without limitation the representations and
warranties set forth in Sections 2 and 3), shall survive the Closing and shall
expire either one (1) year after the Closing Date, for the representations in
Sections 2 and 3 other than those contained in Sections 2.9 and 2.12 through
2.14, or three (3) years after the Closing Date for the representations
contained in Sections 2.9 and 2.12 through 2.14 (the "Expiration Date");
provided, however, that if, at any time prior to the Expiration Date for a
specific representation, any Indemnitee (acting in good faith) delivers to
Seller a written notice alleging the existence of an inaccuracy in or other
Breach of any of such representation and warranty and asserting a claim for
recovery under Section 10.2 based on such alleged inaccuracy or other Breach,
then the claim asserted in such notice shall survive the Expiration Date for
such representation until such time as such claim is fully and finally resolved.
All representations and warranties made by Purchaser shall terminate and expire
as of the Closing, and any liability of Purchaser with respect to such
representations and warranties shall thereupon cease.

              (b) The representations, warranties, covenants and obligations of
Seller and Xenova, and the rights and remedies that may be exercised by the
Indemnitees, shall not be limited or otherwise affected by or as a result of any
information furnished to, or any investigation made by or Knowledge of, any of
the Indemnitees or any of their Representatives.

                                       18
<PAGE>

              (c) For purposes of this Agreement, each statement or other item
of information set forth in the Disclosure Schedule or in any update to the
Disclosure Schedule shall be deemed to be a representation and warranty made by
Seller and Xenova in this Agreement.

          10.2 Indemnification by Xenova.

              (a) Subject to Section 10.3, from and after the Closing Date,
Xenova shall hold harmless and indemnify each of the Indemnitees from and
against, and shall compensate and reimburse each of the Indemnitees for, any
Damages which are directly or indirectly suffered or incurred by any of the
Indemnitees or to which any of the Indemnitees may otherwise become subject at
any time (regardless of whether or not such Damages relate to any third-party
claim) and which arise directly or indirectly from or as a direct or indirect
result of, or are directly or indirectly connected with:

                  (i) any material inaccuracy in or other Breach of any
representation or warranty made by Seller or Xenova in this Agreement;

                 (ii) any material inaccuracy in or Breach of any
representation, warranty, statement, information or provision contained in the
Disclosure Schedule or any supplement to the Disclosure Schedule;

                (iii) the failure of any conveyance instrument or document
delivered by Seller or Xenova in connection with the Closing of the Transactions
to effect its intended conveyance;

                 (iv) Seller's use of the Purchased Assets;

                  (v) Purchaser's performance of research under the Lilly
Contract pursuant to any agreement that may be entered into between Seller,
Xenova and Purchaser relating to the delegation of duties and payment of
research funding to Purchaser (other than arising out of Purchaser's negligence,
recklessness or willful misconduct), as described in Section 1.5;

                 (vi) Seller's employment of the Employees prior to the Closing
Date, and Seller's employment of employees other than Employees who become
employed by Purchaser on the Closing Date;

                (vii) Seller's Breach of its obligations under Section 6.4;

               (viii) any Seller Liabilities other than those which are Assumed
Liabilities or which arise out of Purchased Assets or Assumed Contracts after
the Closing which arise out of or directly relate to events occurring prior to
the Closing Date; or

                 (ix) any Proceeding relating to any Breach, alleged Breach,
Liability or event described in clauses (i) to (viii) above (including any
Proceeding commenced by any Indemnitee for the purpose of enforcing any of its
rights under this Section 10.2).

                                       19
<PAGE>

              (b) Nonexclusivity of Indemnification Remedies. The
indemnification remedies and other remedies provided in this Section 10.2 shall
not be deemed to be exclusive. Accordingly, the exercise by any Person of any of
its rights under this Section 10.2 shall not be deemed to be an election of
remedies and shall not be deemed to prejudice, or to constitute or operate as a
waiver of, any other right or remedy that such Person may be entitled to
exercise (whether under this Agreement, under any other Contract, under any
statute, rule or other Legal Requirement, at common law, in equity or
otherwise).

          10.3 Indemnification Procedures; Defense of Third Party Claims. In the
event of the assertion or commencement by any Person of any claim or Proceeding
(whether against Purchaser, against any other Indemnitee or against any other
Person) with respect to which Xenova may become obligated to indemnify, hold
harmless, compensate or reimburse any Indemnitee pursuant to Section 10.2,
Purchaser shall have the right, at its election, to designate Xenova to assume
the defense of such claim or Proceeding at the sole expense of Xenova. If
Purchaser so elects to designate Xenova to assume the defense of any such claim
or Proceeding:

              (a) Xenova shall proceed to defend such claim or Proceeding in a
diligent manner with counsel reasonably satisfactory to Purchaser;

              (b) Purchaser shall make available to Xenova any non-privileged
documents and materials in the possession of Purchaser that may be necessary to
the defense of such claim or Proceeding;

              (c) Xenova shall keep Purchaser informed of all material
developments and events relating to such claim or Proceeding;

              (d) Purchaser shall have the right to participate in the defense
of such claim or Proceeding at Purchaser's expense;

              (e) Xenova shall not settle, adjust or compromise such claim or
Proceeding without the prior written consent of Purchaser, which consent shall
not be unreasonably withheld; and

              (f) Purchaser may at any time (notwithstanding the prior
designation of Seller to assume the defense of such claim or Proceeding) assume
the defense of such claim or Proceeding.

     If Purchaser does not elect to designate Xenova to assume the defense of
any such claim or Proceeding (or if, after initially designating Xenova to
assume such defense, Purchaser elects to assume such defense), Purchaser may
proceed with the defense of such claim or Proceeding on its own. If Purchaser so
proceeds with the defense of any such claim or Proceeding on its own:

                  (i) all reasonable expenses relating to the defense of such
claim or Proceeding (whether or not incurred by Purchaser) shall be borne and
paid exclusively by Xenova;

                                       20
<PAGE>

                 (ii) Xenova shall make available to Purchaser any documents and
materials in the possession or control of Xenova that may be necessary to the
defense of such claim or Proceeding; and

                (iii) Purchaser shall have the right to settle, adjust or
compromise such claim or Proceeding with the consent of Xenova; provided,
however, that Xenova shall not unreasonably withhold such consent.

          10.4 Exercise of Remedies by Indemnitees Other Than Purchaser. No
Indemnitee (other than Purchaser or any successor thereto or assign thereof)
shall be permitted to assert any indemnification claim or exercise any other
remedy under this Agreement unless Purchaser (or any successor thereto or assign
thereof) shall have consented to the assertion of such indemnification claim or
the exercise of such other remedy.

          10.5 Indemnification by Purchaser.

              (a) Subject to Section 10.6, from and after the Closing Date,
Purchaser shall hold harmless and indemnify each of the Xenova and Seller
Indemnitees from and against, and shall compensate and reimburse each of the
Xenova and Seller Indemnitees for, any Damages which are directly or indirectly
suffered or incurred by any of Xenova and Seller Indemnitees or to which any of
Xenova and Seller Indemnitees may otherwise become subject at any time
(regardless of whether or not such Damages relate to any third-party claim) and
which arise directly or indirectly from or as a direct or indirect result of, or
are directly or indirectly connected with:

                  (i) any material inaccuracy in or other Breach of any
representation or warranty made by Purchaser in this Agreement;

                 (ii) Purchaser's use of the Purchased Assets and employment of
the individuals set forth on Exhibit A after the Closing Date;

                (iii) any Assumed Liabilities; or

                 (iv) any Proceeding relating to any Breach, alleged Breach,
Liability or event described in clauses (i) to (iii) above (including any
Proceeding commenced by any Indemnitee for the purpose of enforcing any of its
rights under this Section 10.2).

Purchaser's obligations to indemnify and hold Seller and Xenova harmless from
Liabilities arising from the Britannia Agreement are set forth in the Assignment
and Assumption Agreement and Consent to be executed by the Parties and Britannia
pursuant to Section 1.6.

          (b) Nonexclusivity of Indemnification Remedies. The indemnification
remedies and other remedies provided in this Section 10.5 shall not be deemed to
be exclusive. Accordingly, the exercise by any Person of any of its rights under
this Section 10.5 shall not be deemed to be an election of remedies and shall
not be deemed to prejudice, or to constitute or operate as a waiver of, any
other right or remedy that such Person may be entitled

                                       21
<PAGE>

to exercise (whether under this Agreement, under any other Contract, under any
statute, rule or other Legal Requirement, at common law, in equity or
otherwise).

          10.6 Indemnification Procedures; Defense of Third Party Claims. In the
event of the assertion or commencement by any Person of any claim or Proceeding
(whether against Xenova and Seller, against any other Indemnitee or against any
other Person) with respect to which Purchaser may become obligated to indemnify,
hold harmless, compensate or reimburse any Xenova or Seller Indemnitee pursuant
to Section 10.6, Purchaser shall have the right, at its election, to assume the
defense of such claim or Proceeding at the sole expense of Purchaser. If
Purchaser so elects to assume the defense of any such claim or Proceeding:

              (a) Purchaser shall proceed to defend such claim or Proceeding in
a diligent manner with counsel reasonably satisfactory to Xenova and Seller;

              (b) Xenova and Seller shall make available to Purchaser a any non-
privileged documents and materials in the possession of Xenova and Seller that
may be necessary to the defense of such claim or Proceeding;

              (c) Purchaser shall keep Xenova and Seller informed of all
material developments and events relating to such claim or Proceeding;

              (d) Xenova and Seller shall have the right to participate in the
defense of such claim or Proceeding at their own expense; and

              (e) Purchaser shall not settle, adjust or compromise such claim or
Proceeding without the prior written consent of Xenova and Seller, which consent
shall not be unreasonably withheld.

     If Purchaser does not elect to assume the defense of any such claim or
Proceeding, Xenova and Seller may proceed with the defense of such claim or
Proceeding on their own. If Xenova and Seller so proceeds with the defense of
any such claim or Proceeding on their own:

                  (i) all reasonable expenses relating to the defense of such
claim or Proceeding (whether or not incurred by Xenova and Seller) shall be
borne and paid exclusively by Purchaser;

                 (ii) Purchaser shall make available to Xenova and Seller any
documents and materials in the possession or control of Purchaser that may be
necessary to the defense of such claim or Proceeding; and

                (iii) Xenova and Seller shall have the right to settle, adjust
or compromise such claim or Proceeding with the consent of Purchaser; provided,
however, that Purchaser shall not unreasonably withhold such consent.

                                       22
<PAGE>

     11.  Miscellaneous Provisions

          11.1  Further Assurances. Each party hereto shall execute and/or cause
to be delivered to each other party hereto such instruments and other documents,
and shall take such other actions, as such other party may reasonably request
(prior to, at or after the Closing) for the purpose of carrying out or
evidencing any of the Transactions.

          11.2   Payment of all Federal and State Taxes due on Sale of Assets by
Seller. Seller shall pay in a timely manner all Taxes resulting from the sale of
the Purchased Assets pursuant to this Agreement. Seller shall not make a
distribution of any of the proceeds of the sale to its members until Seller
shall have determined its tax liability resulting from the sale and shall have
either paid or provided adequate reserve for payment of such Tax.
Notwithstanding, Purchaser shall within thirty (30) days after receiving and
invoice therefor from Seller reimburse to Seller one-half (1/2) of sales taxes
due on the sale of Purchased Assets pursuant to this Agreement.

          11.3   Fees and Expenses.  Subject to Section 6, each party to this
Agreement shall bear and pay all fees, costs and expenses (including legal fees
and accounting fees) that have been incurred or that are incurred in the future
by such party in connection with the transactions contemplated by this
Agreement, including all fees, costs and expenses incurred by such party in
connection with or by virtue of:

                 (a) the negotiation, preparation and review of this Agreement
(including the Disclosure Schedule), the other Transactional Agreements and all
certificates, opinions and other instruments and documents delivered or to be
delivered in connection with the Transactions; and

                 (b) the consummation and performance of the Transactions.

          11.4   Attorneys' Fees.  If any legal action or other legal proceeding
relating to any of the Transactional Agreements or the enforcement of any
provision of any of the Transactional Agreements is brought against any party
hereto, the prevailing party shall be entitled to recover reasonable attorneys'
fees, costs and disbursements (in addition to any other relief to which the
prevailing party may be entitled).

          11.5   Notices. Any notice or other communication required or
permitted to be delivered to any party under this Agreement shall be in writing
and shall be deemed properly delivered, given and received when delivered (by
hand, by registered mail, by courier or express delivery service or by
telecopier) to the address or telecopier number set forth beneath the name of
such party below (or to such other address or telecopier number as such party
shall have specified in a written notice given to the other parties hereto):

                                       23
<PAGE>

          if to Seller:

               MetaXen LLC
               280 East Grand Avenue
               South San Francisco, CA  94080
               Attention:  Michael Ross, Ph.D.
               Facsimile:  (650) 553-8101

          with a copy to:
          ---------------

               Wilson, Sonsini, Goodrich & Rosati
               650 Page Mill Road
               Palo Alto, CA 94306
               Attention:  Michael J. O'Donnell, Esq.
               Facsimile:  (650) 493-6811

          if to Purchaser:

               Exelixis Pharmaceuticals, Inc.
               260 Littlefield Avenue
               South San Francisco, CA  94080
               Attention:  George Scangos
               Facsimile:  (650) 825-2205

          with a copy to:
          --------------

               Cooley Godward LLP
               Five Palo Alto Square
               3000 El Camino Real
               Palo Alto, CA  94306
               Attention: Deborah A Marshall, Esq.
               Facsimile:  (650) 857-0663

          if to Xenova:

               Xenova Group plc
               240 Bath Road
               Slough
               Berkshire SL1 4EF
               England
               Attention:  Daniel Abrams
               Facsimile: 44 1753 706638

                                       24
<PAGE>

          with a copy to:
          --------------

               Brobeck, Phleger and Harrison, LLP
               Two Embarcadero Place
               2200 Geng Road
               Palo Alto, CA 94303
               Attention:  Stephan Dolezalek, Esq.
               Facsimile:  (650) 496-2885

          11.6   Publicity.

                 (a) Purchaser acknowledges that Xenova intends to issue a press
release relating to the Transactions promptly after the Effective Date to comply
with its reporting obligations as a publicly held company, and Xenova agrees
that Purchaser may also issue a press release relating to the Transactions
concurrently with or promptly following any such press release by Xenova. Any
such press release may describe the general nature of the Transactions but shall
not specify in detail the terms of this Agreement. Additionally, Purchaser may
release publicly a statement regarding its actual or intended acquisition of the
Purchased Assets and its employment of the Employees in connection with any
statements Purchaser publicly discloses relating to corporate partnerships
involving combinatorial chemistry technology.

                 (b) On and at all times after the Closing Date, except as
otherwise provided in this Section 11.6, no press release or other publicity
concerning any of the Transactions shall be issued or otherwise disseminated by
or on behalf of Xenova and Seller or any of Seller's members, and Xenova and
Seller shall continue to keep the existence and terms of this Agreement and the
other Transactional Agreements strictly confidential; and Seller and Xenova
shall keep strictly confidential, and shall not use or disclose to any other
Person, any non-public document or other information in Seller's or Xenova's
possession that relates directly or indirectly to the Transactions, Purchaser or
any affiliate of Purchaser.

                 (c) Xenova and Purchaser shall provide to each other a draft of
any press release permitted to be made pursuant to this Section 11.6 for review
and comment by the other party at least one (1) business day in advance of the
date upon which such release shall be made.

          11.7   Time of the Essence.  Time is of the essence of this Agreement.

          11.8   Headings.  The headings contained in this Agreement are for
convenience of reference only, shall not be deemed to be a part of this
Agreement and shall not be referred to in connection with the construction or
interpretation of this Agreement.

                                       25
<PAGE>

          11.9      Counterparts.  This Agreement may be executed in several
counterparts, each of which shall constitute an original and all of which, when
taken together, shall constitute one agreement.

          11.10     Governing Law; Dispute Resolution.

          (a) This Agreement shall be construed in accordance with, and governed
in all respects by, the internal laws of the State of California (without giving
effect to principles of conflicts of laws).

          (b) Any legal action or other legal proceeding relating to this
Agreement or the enforcement of any provision of this Agreement may be brought
or otherwise commenced in any state or federal court located in the County of
San Mateo, California. Each party to this Agreement:

               (i)   expressly and irrevocably consents and submits to the
     jurisdiction of each state and federal court located in the County of San
     Mateo, California (and each appellate court located in the State of
     California) in connection with any such legal proceeding;

               (ii)  agrees that each state and federal court located in the
     County of San Mateo, California shall be deemed to be a convenient forum;
     and

               (iii) agrees not to assert (by way of motion, as a defense or
     otherwise), in any such legal proceeding commenced in any state or federal
     court located in the County of San Mateo, California, any claim that such
     party is not subject personally to the jurisdiction of such court, that
     such legal proceeding has been brought in an inconvenient forum, that the
     venue of such proceeding is improper or that this Agreement or the subject
     matter of this Agreement may not be enforced in or by such court.

          (c) Xenova and the Seller agree that, if any Proceeding is commenced
against any Indemnitee by any Person in or before any court or other tribunal
anywhere in the world, then such Indemnitee may proceed against Xenova and the
Seller in or before such court or other tribunal with respect to any
indemnification claim or other claim arising directly or indirectly from or
relating directly or indirectly to such Proceeding or any of the matters alleged
therein or any of the circumstances giving rise thereto.

          (d) Xenova irrevocably constitutes and appoints Brobeck, Phleger &
Harrison, LLP (attn: J. Stephan Dolezalek)  as ("Xenova's Representative") as
its agent to receive service of process in connection with any legal proceeding
relating to this Agreement or the enforcement of any provision of this
Agreement.

          11.11     Successors and Assigns; Assignment.  This Agreement shall be
binding upon: Seller and Purchaser and their respective successors and permitted
assigns (if any) and Xenova and their respective personal representatives,
executors, administrators, estates, heirs, successors and permitted assigns (if
any). This Agreement shall inure to the benefit of: Seller; Xenova and their
respective personal representatives, executors, administrators, estates and
heirs;

                                       26
<PAGE>

Purchaser; the other Indemnitees and the respective successors and permitted
assigns (if any) of the foregoing. Purchaser may freely assign any or all of its
rights under this Agreement (including its indemnification rights under Section
10), in whole or in part, to any other Person.

          11.12   Waiver.

                  (a) No failure on the part of any Person to exercise any
power, right, privilege or remedy under this Agreement, and no delay on the part
of any Person in exercising any power, right, privilege or remedy under this
Agreement, shall operate as a waiver of such power, right, privilege or remedy;
and no single or partial exercise of any such power, right, privilege or remedy
shall preclude any other or further exercise thereof or of any other power,
right, privilege or remedy.

                  (b) No Person shall be deemed to have waived any claim arising
out of this Agreement, or any power, right, privilege or remedy under this
Agreement, unless the waiver of such claim, power, right, privilege or remedy is
expressly set forth in a written instrument duly executed and delivered on
behalf of such Person; and any such waiver shall not be applicable or have any
effect except in the specific instance in which it is given.

          11.13   Amendments. This Agreement may not be amended, modified,
altered or supplemented other than by means of a written instrument duly
executed and delivered on behalf of Purchaser and the Agent.

          11.14   Severability. In the event that any provision of this
Agreement, or the application of any such provision to any Person or set of
circumstances, shall be determined to be invalid, unlawful, void or
unenforceable to any extent, the remainder of this Agreement, and the
application of such provision to Persons or circumstances other than those as to
which it is determined to be invalid, unlawful, void or unenforceable, shall not
be impaired or otherwise affected and shall continue to be valid and enforceable
to the fullest extent permitted by law.

          11.15   Parties in Interest. Except for the provisions of Section 10
hereof, none of the provisions of this Agreement is intended to provide any
rights or remedies to any Person other than the parties hereto and their
respective successors and assigns (if any).

          11.16   Entire Agreement. The Transactional Agreements (including all
schedules and exhibits attached thereto) set forth the entire understanding of
the parties relating to the subject matter thereof and supersede all prior
agreements and understandings among or between any of the parties relating to
the subject matter hereof.

          11.17   Construction.

                  (a) For purposes of this Agreement, whenever the context
requires: the singular number shall include the plural, and vice versa; the
masculine gender shall include the feminine and neuter genders; the feminine
gender shall include the masculine and neuter genders; and the neuter gender
shall include the masculine and feminine genders.

                                       27
<PAGE>

        (b) The parties hereto agree that any rule of construction to the effect
that ambiguities are to be resolved against the drafting party shall not be
applied in the construction or interpretation of this Agreement.

        (c) As used in this Agreement, the words "include" and "including," and
variations thereof, shall not be deemed to be terms of limitation, but rather
shall be deemed to be followed by the words "without limitation."

        (d) Except as otherwise indicated, all references in this Agreement to
"Sections" and "Exhibits" are intended to refer to Sections of this Agreement
and Exhibits to this Agreement.

     The parties hereto have caused this Agreement to be executed and delivered
as of July 11, 1999.

"Purchaser":                  Exelixis Pharmaceuticals, Inc.

                              By: /s/ George Scangos
                                 ________________________________________
                                    Name:  George Scangos
                                    Title: Chief Executive Officer

"Seller":                     MetaXen LLC

                              By: /s/ David Oxlade
                                 ________________________________________
                                    Name:  David Oxlade
                                    Title: Interim President

"Xenova":                     Xenova Group plc

                                      /s/ Daniel Abrams
                                    -------------------------------
                                    Name: Daniel Abrams
                                    Title:  Chief Financial Officer

                                       28
<PAGE>

                                   Exhibit A

                                   Employees

Aftab, Dana
Brown, S. David
Brzezinski, Linda
Buckley, Douglas
Canne, Lynne
Chan, Jocelyn
Epshteyn, Sergey
Ewing, Todd
Galan, Adam
Graham, Hugh
Hammonds, R. Glenn
Hanel, Art
Kearney, Patrick
Leahy, James
Matthews, David
Phife, David
Prisbylla, Michael
Ricafort, Lourdes
Sage, Carleton
Singh, Rahul
Stout, Thomas
Tesfai, Zerom
Trainor, Michael
Wu, Pengguang

                                       29
<PAGE>

                                   Exhibit B

                              CERTAIN DEFINITIONS

          For purposes of the Agreement (including this Exhibit B):

     Affiliate.  "Affiliate" shall mean and include:

                    (a)  any member of Seller;

                    (b)  any record or beneficial holder of outstanding
securities of Xenova or Seller;

                    (c)  any sibling, uncle, aunt, niece or nephew of any Person
described in clause (a) or (b);

                    (d)  any ancestor or lineal descendant of any person
described in clauses (a), (b) or (c);

                    (e)  any current or former spouse of any person described in
clauses, (a), (b), (c) or (d) or any person who is a member of the same
household of the person described in clauses (a), (b), (c) or (d) or who has
resided with such person for more than 10 days in any calendar year;

                    (f)  any ancestor or lineal descendant of any person
described in clauses (a), (b), (c), (d) or (e); or

                    (g)  any Person in which any of the foregoing have a direct
or indirect interest.

     Agreement.  "Agreement" shall mean the Asset Purchase Agreement to which
this Exhibit B is attached (including without limitation the Disclosure Schedule
and any other exhibits, schedules or attachments thereto), as it may be amended
from time to time.

     Assumed Contract.  "Assumed Contract" shall have the meaning set forth in
Section 1.1(b) to the Agreement.

     Assumed Liability.  "Assumed Liability" shall have the meaning set forth in
Section 1.7 to the Agreement.

     Best Efforts.  "Best Efforts" shall mean the efforts that a prudent Person
desiring to achieve a particular result would use in order to ensure that such
result is achieved as expeditiously as possible.

     Breach.  There shall be deemed to be a "Breach" of a representation,
warranty, covenant, obligation or other provision if there is or has been (a)
any inaccuracy in or Breach of, or any

                                      A-1
<PAGE>

failure to comply with or perform, such representation, warranty, covenant,
obligation or other provision, or (b) any claim (by any Person) or other
circumstance that is inconsistent with such representation, warranty, covenant,
obligation or other provision; and the term "Breach" shall be deemed to refer to
any such inaccuracy, Breach, failure, claim or circumstance.

     Business.  "Business" shall have the meaning specified in Section A of the
Recitals to the Agreement.

     Claims.  "Claims" shall mean and include all past, present and future
disputes, claims, controversies, demands, rights, obligations, liabilities,
actions and causes of action of every kind and nature, including: (i) any
unknown, unsuspected or undisclosed claim; (ii) any claim or right that may be
asserted or exercised by a member in such member's capacity as a member or
employee of Seller or in any other capacity; and (iii) any claim, right or claim
of action based upon any breach of any express, implied, oral or written
contract or agreement.

     Closing.  "Closing" shall have the meaning specified in Section 1.11 of the
Agreement.

     Closing Date.  "Closing Date" shall have the meaning specified in Section
1.11 of the Agreement.

     Consent.  "Consent" shall mean any approval, consent, ratification,
permission, waiver or authorization (including any Governmental Authorization).

     Contract.  "Contract" shall mean any written, oral, implied or other
agreement, contract, understanding, arrangement, instrument, note, guaranty,
indemnity, representation, warranty, deed, assignment, power of attorney,
certificate, purchase order, sales order, work order, insurance policy, benefit
plan, commitment, covenant, assurance or undertaking of any nature.

     Damages.  "Damages" shall include any loss, damage, injury, lost
opportunity, Liability, claim, demand, settlement, judgment, award, fine,
penalty, Tax, fee (including any legal fee, expert fee, accounting fee or
advisory fee), charge, cost (including any cost of investigation) or expense of
any nature.

     Disclosure Schedule.  "Disclosure Schedule" shall mean the schedule (dated
as of the date of the Agreement) delivered to Purchaser on behalf of Seller and
Xenova, a copy of which is attached to the Agreement and incorporated in the
Agreement by reference.

     Encumbrance.  "Encumbrance" shall mean any lien, pledge, hypothecation,
charge, mortgage, security interest, encumbrance, equity, trust, equitable
interest, claim, preference, right of possession, lease, tenancy, license,
encroachment, covenant, infringement, interference, Order, proxy, option, right
of first refusal, preemptive right, community property interest, legend, defect,
impediment, exception, reservation, limitation, impairment, imperfection of
title, condition or restriction of any nature (including any restriction on the
voting of any security, any restriction on the transfer of any security or other
asset, any restriction on the receipt of any income derived from any asset, any
restriction on the use of any asset and any restriction on the possession,
exercise or transfer of any other attribute of ownership of any asset).

                                      A-2
<PAGE>

     Entity.  "Entity" means any corporation (including any non-profit
corporation), general partnership, limited partnership, limited liability
partnership, joint venture, estate, trust, company (including any limited
liability company or joint stock company), firm or other enterprise,
association, organization or entity.

     Environmental Laws.  "Environmental Laws" means all applicable federal,
state and local laws relating to pollution, storage, releases or threatened
releases of pollutants, contaminants, radioactive substances, chemicals or
industrial, toxic, hazardous or petroleum-based substances or wastes ("Waste")
into the environment (including, without limitation, ambient air, surface water,
ground water, land surface or subsurface strata) or otherwise relating to the
manufacture, processing, distribution, use, treatment, storage, disposal,
transport or handling of Waste including, without limitation, the Clean Water
Act, the Clean Air Act, the Resource Conservation and Recovery Act, the Toxic
Substances Control Act and the Comprehensive Environmental Response Compensation
Liability Act ("CERCLA"), as amended, and their state and local counterparts.

     Expiration Date.  "Expiration Date" shall have the meaning set forth in
Section 10.1(a) to the Agreement.

     Governmental Authorization.  "Governmental Authorization" shall mean any:

             (h) permit, license, certificate, franchise, concession, approval,
consent, ratification, permission, clearance, confirmation, endorsement, waiver,
certification, designation, rating, registration, qualification or authorization
that is, has been or may in the future be issued, granted, given or otherwise
made available by or under the authority of any Governmental Body or pursuant to
any Legal Requirement; or

             (i) right under any Contract with any Governmental Body.

     Governmental Body.  "Governmental Body" shall mean any:

             (j) nation, principality, state, commonwealth, province, territory,
county, municipality, district or other jurisdiction of any nature;

             (k) federal, state, local, municipal, foreign or other government;

             (l) governmental or quasi-governmental authority of any nature
(including any governmental division, subdivision, department, agency, bureau,
branch, office, commission, council, board, instrumentality, officer, official,
representative, organization, unit, body or Entity and any court or other
tribunal);

             (m) multi-national organization or body; or

             (n) individual, Entity or body exercising, or entitled to exercise,
any executive, legislative, judicial, administrative, regulatory, police,
military or taxing authority or power of any nature.

                                      A-3
<PAGE>

     Indemnitees.  "Indemnitees" shall mean the following Persons:

             (o) Purchaser;

             (p) Purchaser's current and future affiliates;

             (q) The respective Representatives of the Persons referred to in
clauses "(a)" and "(b)" above; and

             (r) The respective successors and assigns of the Persons referred
to in clauses "(a)", "(b)" and "(c)" above.

     Knowledge.  An individual shall be deemed to have "Knowledge" of a
particular fact or other matter if:

             (s) Such individual is actually aware of such fact or other matter;
or

             (t) A prudent individual could be expected to discover or otherwise
become aware of such fact or other matter in the course of conducting a diligent
and comprehensive investigation concerning the truth or existence of such fact
or other matter.

An Entity shall be deemed to have "Knowledge" of a particular fact or other
matter if any officer, employee or other Representative of such Entity has
Knowledge of such fact or other matter.

     Legal Requirement.  "Legal Requirement" shall mean any federal, state,
local, municipal, foreign or other law, statute, legislation, constitution,
principle of common law, resolution, ordinance, code, edict, decree,
proclamation, treaty, convention, rule, regulation, ruling, directive,
pronouncement, requirement, specification, determination, decision, opinion or
interpretation that is, has been or may in the future be issued, enacted,
adopted, passed, approved, promulgated, made, implemented or otherwise put into
effect by or under the authority of any Governmental Body.

     Liability.  "Liability" shall mean any debt, obligation, duty or liability
of any nature (including any unknown, undisclosed, unmatured, unaccrued,
unasserted, contingent, indirect, conditional, implied, vicarious, derivative,
joint, several or secondary liability), regardless of whether such debt,
obligation, duty or liability would be required to be disclosed on a balance
sheet prepared in accordance with GAAP and regardless of whether such debt,
obligation, duty or liability is immediately due and payable.

     Order.  "Order" shall mean any:

             (u) order, judgment, injunction, edict, decree, ruling,
pronouncement, determination, decision, opinion, verdict, sentence, subpoena,
writ or award that is, has been or may in the future be issued, made, entered,
rendered or otherwise put into effect by or under the

                                      A-4
<PAGE>

authority of any court, administrative agency or other Governmental Body or any
arbitrator or arbitration panel; or

             (v) Contract with any Governmental Body that is, has been or may in
the future be entered into in connection with any Proceeding.

     Ordinary Course of Business.  An action taken by or on behalf of an Entity
shall not be deemed to have been taken in the "Ordinary Course of Business"
unless:

             (w) such action is recurring in nature, is consistent with an
Entity's past practices and is taken in the ordinary course of such Entity's
normal day-to-day operations;

             (x) such action is taken in accordance with sound and prudent
business practices; and

             (y) such action is not required by applicable law or governing
documents to be authorized by an Entity's stockholders, members, board of
directors or similar body, or committee of such Entity's board of directors or
similar body and does not require any other separate or special authorization of
any nature.

     Person.  "Person" shall mean any individual, Entity or Governmental Body.

     Pre-Closing Period. shall mean the period from the date of the Agreement
through the Closing Date.

     Proceeding.  "Proceeding" shall mean any action, suit, litigation,
arbitration, proceeding (including any civil, criminal, administrative,
investigative or appellate proceeding and any informal proceeding), prosecution,
contest, hearing, inquiry, inquest, audit, examination or investigation that is,
has been or may in the future be commenced, brought, conducted or heard by or
before, or that otherwise has involved or may involve, any Governmental Body or
any arbitrator or arbitration panel.

     Proprietary Asset.  "Proprietary Asset" shall mean any (a) patent, patent
application, trademark (whether registered or unregistered and whether or not
relating to a published work), trademark application, trade name, fictitious
business name, service mark (whether registered or unregistered), service mark
application, trade secret, know-how, franchise, system, computer software,
invention, design, blueprint, proprietary product, technology, proprietary right
or other intellectual property right or intangible asset; or (b) right to use or
exploit any of the foregoing.

     Purchased Asset.  "Purchased Asset" shall have the meaning set forth in
Section 1.1 to the Agreement.

     Purchaser.  "Purchaser" shall have the meaning set forth in the
introductory paragraph to the Agreement.

     Representatives.  "Representatives" shall mean officers, directors,
employees, agents,

                                      A-5
<PAGE>

attorneys, accountants, advisors and representatives.  Xenova and all other
Related Parties shall be deemed to be "Representatives" of Seller.

     Seller.  "Seller" shall have the meaning specified in the introductory
paragraph of the Agreement.

     Seller Contract.  "Seller Contract" shall mean any Contract:

             (z)     to which Seller is a party;

             (aa)    by which Seller or any of its assets are or may become
bound or under which Seller has, or may become subject to, any obligation; or

             (bb)    under which Seller has or is likely to acquire any right or
interest.

     Xenova.  "Xenova" shall have the meaning specified in the introductory
paragraph of the Agreement.

     Tax.  "Tax" shall mean any tax (including any income tax, franchise tax,
capital gains tax, estimated tax, gross receipts tax, value-added tax, surtax,
excise tax, ad valorem tax, transfer tax, stamp tax, sales tax, use tax,
property tax, business tax, occupation tax, inventory tax, occupancy tax,
withholding tax or payroll tax), levy, assessment, tariff, impost, imposition,
toll, duty (including any customs duty), deficiency or fee, and any related
charge or amount (including any fine, penalty or interest), that is, has been or
may in the future be (a) imposed, assessed or collected by or under the
authority of any Governmental Body, or (b) payable pursuant to any tax-sharing
agreement or similar Contract.

     Tax Return.  "Tax Return" shall mean any return (including any information
return), report, statement, declaration, estimate, schedule, notice,
notification, form, election, certificate or other document or information that
is, has been or may in the future be filed with or submitted to, or required to
be filed with or submitted to, any Governmental Body in connection with the
determination, assessment, collection or payment of any Tax or in connection
with the administration, implementation or enforcement of or compliance with any
Legal Requirement relating to any Tax.

     Transactional Agreements.  "Transactional Agreements" shall mean the
Agreement and all other documents or agreements contemplated by Section 1.

     Transactions.  "Transactions" shall mean (a) the execution and delivery of
the respective Transactional Agreements and (b) all of the transactions
contemplated by the respective Transactional Agreements.

                                      A-6

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