<PAGE>

                                                                    Exhibit 10.8
                                                                    ------------

                   RESEARCH AND COMMERCIALIZATION AGREEMENT

     THIS RESEARCH AND COMMERCIALIZATION AGREEMENT (the "Agreement"), effective
as of September 21, 1999 (the "Effective Date"), is entered into by and among
Medarex, Inc., a New Jersey corporation, with a principal place of business at
1545 Route 22 East, Annandale, New Jersey 08801 and GenPharm International,
Inc., a California corporation and wholly-owned subsidiary of Medarex, Inc.,
with a principal place of business at 2350 Qume Drive, San Jose, California
95131-1807 ("Medarex", as defined below), and Amgen Inc., a Delaware
corporation, with a principal place of business at One Amgen Center Drive,
Thousand Oaks, California 91320-1799 ("Amgen", as defined below).

                                  BACKGROUND

     A.   Medarex is the sole and exclusive owner of certain transgenic Mice (as
defined below) useful for the preparation of fully human monoclonal antibodies;

     B.   Medarex has the right to conduct research with such Mice and related
technology;

     C.   Amgen desires Medarex to conduct research with the Mice to generate
fully human monoclonal antibodies to certain Antigens (as defined below), on the
terms and conditions herein;

     D.   Amgen further desires to analyze, characterize, evaluate the utility
of and conduct research with such fully human monoclonal antibodies to such
certain Antigens, on the terms and conditions herein; and

     E.   Amgen wishes to acquire from Medarex an option to acquire a commercial
license for such fully human monoclonal antibodies, on the terms and conditions
herein.

     NOW, THEREFORE, Medarex and Amgen agree as follows:

     1.   DEFINITIONS

     1.1  "Affiliate" means any corporation or other entity which is directly or
           ---------
indirectly controlling, controlled by or under common control with a party. For
the purpose of this definition, "control" shall mean the direct or indirect
ownership of fifty percent (50%) or more of the outstanding shares or other
voting rights of the subject entity to elect directors, or if not meeting the
preceding, any entity owned or controlled by or owning or controlling at the
maximum control or ownership right permitted in the country where such entity
exists.
<PAGE>

     1.2  "Amgen" shall mean Amgen Inc. and Affiliates of Amgen Inc.
           -----

     1.3  "Amgen Technology" shall mean the Amgen Patent Rights and Amgen Know
           ----------------
 How.

          1.3.1  [*****]

          1.3.2  "Amgen Patent Rights" shall mean all Patent Rights which
                  -------------------
claim Amgen Know How and which are owned or Controlled by Amgen as of the
Effective Date or during the term of the Agreement.

     1.4  "Antibody(ies)" shall mean one or more fully human monoclonal
           -------------
antibodies having a binding affinity for an Antigen (a) delivered by Medarex to
Amgen pursuant to this Agreement [*****].

     1.5  "Antibody Cell(s)" shall mean one or more cells expressing or
           ----------------
secreting Antibody(ies) and/or containing Genetic Material(s) which encode
Antibody(ies).

     1.6  "Antigen(s)" shall mean one or more specifically defined chemical or
           ----------
biological entities, as set forth in a mutually agreed written description in
Exhibit B or Exhibit C which shall include DNA sequences and amino acid
sequences, where available, which Exhibits shall be amended from time to time in
accordance with Section 2.1.2. [*****].

     1.7  "Approval" shall mean all approvals, licenses, registrations and
           --------
authorizations of all governmental agencies in a country necessary for the
distribution, importation, manufacture, production, use, storage, transport or
sale of a Product(s) in the applicable country.

     1.8  "Biological License Application" or "BLA" shall mean a Biological
           ------------------------------      ---
License Application as defined in the U.S. Food, Drug and Cosmetic Act and the
regulations promulgated thereunder, and any corresponding foreign application,
registration or certification to obtain marketing approval of a human biological
therapeutic product.

     1.9  [*****].

          1.9.1  [*****].

          1.9.2  [*****].

     1.10  "Commercially Reasonable Efforts" shall mean [*****].
            -------------------------------

***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       2
<PAGE>

     1.11  "Confidential Information" shall mean any proprietary or confidential
            ------------------------
information of a party which (i) if disclosed in tangible form hereunder, shall
be marked as "Confidential" at the time it shall be delivered to the receiving
party, or (ii) if disclosed orally, shall be identified as confidential or
proprietary when disclosed and shall be confirmed in writing within thirty (30)
days by the disclosing party.

     1.12  "Control" or "Controlled" shall mean possession of the ability to
            -------      ----------
grant the licenses or sublicenses as provided for herein without violating the
terms of any agreement or other arrangement with any Third Party.

     1.13  "Cross-License Agreement" shall mean that certain Cross License
            -----------------------
Agreement entered into by and between Abgenix, Inc., Cell Genesys, Inc., Japan
Tobacco Inc., Xenotech L.P., and GenPharm International, Inc., effective as of
March 26, 1997, as the same may be amended from time to time.

     1.14  "Evaluation" shall mean, with respect to an Antibody(ies) to a
            ----------
particular Antigen listed in Exhibit C, the activities conducted by Amgen
pursuant to Article 3 during the applicable Evaluation Period to determine
whether Amgen wishes to obtain a commercial license with respect to Product(s)
directed against such Antigen.

     1.15  "Evaluation Period" shall mean, with respect to a particular
            -----------------
Evaluation, the period from the initiation of such Evaluation as set forth in
Section 3.3 until the earlier of the termination of (i) the period set forth in
Section 3.3 or (ii) [*****].

     1.16  "Field of Use" shall mean all uses of Product(s), including all human
            ------------
therapeutic, prophylactic and diagnostic uses of Product(s).

     1.17  "Genetic Material(s)" shall mean the nucleotide sequences encoding an
            -------------------
entire Antibody (or a fragment of an entire Antibody containing that portion of
the Antibody conferring binding specificity for an Antigen) [*****].

     1.18  "Immunization" shall mean, with respect to an Antigen listed in
            ------------
Exhibit C, the activities conducted by Medarex pursuant to Article 2 during the
applicable Immunization Period for the generation and characterization of
Antibody(ies) to such Antigen.

     1.19  "Immunization Period" shall mean, with respect to a particular
            -------------------
Immunization, the period from the initiation of such Immunization as set forth
in Section 2.3 until the earlier of the termination of (i) the period set forth
in Section 2.3 or (ii) [*****].


***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       3
<PAGE>

     1.20  "IND" shall mean an Investigational New Drug application, as
            ---
defined in the U.S. Food, Drug and Cosmetic Act and the regulations promulgated
thereunder, or any corresponding foreign application, registration or
certification.

     1.21  [*****].

     1.22  [*****].

           1.22.1  [*****].

           1.22.2  [*****].

     1.23  "Licensed Technology" shall mean the Licensed Know How and Licensed
            -------------------
Patent Rights.

           1.23.1  [*****].

           1.23.2  [*****].

     1.24  "Major Market" shall mean each of [*****].
            ------------

     1.25  "Medarex" shall mean Medarex, Inc., GenPharm International, Inc.,

B.V., and any Affiliate of Medarex whose principal business activity is the
development and/or commercialization of human or humanized antibodies for human
therapeutic use.

     1.26  "Medarex Technology" shall mean the Medarex Patent Rights and
            ------------------
Medarex Know How.

          1.26.1  "Medarex Know How" shall mean the Confidential Information and
                   ----------------
materials, including without limitation, biological materials, technical data,
protocols, methods and processes, owned or Controlled by Medarex during the term
of the Agreement necessary for the exercise of an invention claimed by the
Medarex Patent Rights. For the avoidance of doubt, the Medarex Know How shall
not include any Medarex Patent Rights.

          1.26.2  "Medarex Patent Rights" shall mean all Patent Rights [*****],
                   ---------------------
owned or Controlled by Medarex during the term of the Agreement, in each case,
which claim an invention necessary to make, have made, import, have imported,
use, offer for sale and sell Product(s), specifically including all Patent
Rights Controlled by Medarex under the MRC and Cross-License Agreements.

***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       4
<PAGE>

     1.27  "Mice" shall mean immunizable transgenic mice containing
            ----
unrearranged human immunoglobulin genes which are owned or Controlled by Medarex
as of the Effective Date or during the term of the Agreement but not including
transgenic mice in-licensed by Medarex after the Effective Date (other than
improvements of transgenic mice owned or Controlled by Medarex as of the
Effective Date). "Mouse" shall mean one of the Mice.
                  ------

     1.28  "MRC Agreement" shall mean that certain License Agreement entered
            -------------
by and between (i) the Medical Research Council, Institute of Animal Physiology
and Genetics Immunization of Babraham Hall and Marianne Bruggemann and (ii)
GenPharm International, Inc., effective October 1, 1993, as amended.

     1.29  "Net Sales" shall mean [*****].
            ---------

     1.30  "Patent Rights" shall mean all United States and foreign patents
            -------------
(including all utility, model and design patents and certificates of invention,
reissues, extensions, substitutions, confirmations, re-registrations, re-
examinations, revalidations, and patents of addition) and patent applications
(including without limitation, all continuations, continuations-in-part and
divisions thereof).

     1.31  "Pivotal Clinical Trial" shall mean a clinical trial or a
            ----------------------
comparable trial which, [*****], shall be intended [*****] to provide
statistically significant evidence of the safety and efficacy of a Product in
humans sufficient to obtain regulatory approval for the sale of such Product.

     1.32  "Product(s)" shall mean, with respect to an Antigen listed in
            ----------
Exhibit C, a composition or compositions, with each composition comprising one
or more Antibody(ies), [*****].

     1.33  "Regulatory Filings" shall mean, collectively, INDs, BLAs,
            ------------------
establishment license applications (ELAs) and drug master files (DMFs) or any
other similar filings (including any foreign equivalents and further including
any related correspondence and discussions) as may be required by the FDA or
equivalent foreign regulatory agencies for the clinical testing, manufacture or
sale of a Product.

     1.34  "Research Plan" shall mean a mutually-agreed-upon written plan
            -------------
describing the generation and characterization of Antibody(ies) to an Antigen
listed in Exhibit C and to related activities, which plan shall be carried out
by Medarex solely in furtherance of an Immunization and Evaluation. Each
Research Plan for each Immunization and Evaluation shall be attached to and made
part of Exhibit A of this Agreement.

***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       5
<PAGE>

     1.35  "Royalty" or "Royalties" shall mean the royalty or royalties
            -------      ---------
payable by Amgen to Medarex pursuant to Section 5.6.

     1.36  "Sublicensee" shall mean a Third Party to whom Amgen shall have
            -----------
granted a license or sublicense to make, have made, import, use, sell, offer for
sale or otherwise exploit a Product(s) in the Territory. As used in this
Agreement, "Sublicensee" shall also include a Third Party which Amgen or a
Sublicensee shall have granted the right to distribute one or more Product(s)
[*****].

     1.37  "Territory" shall mean all countries of the world.
            ---------

     1.38  "Third Party" shall mean any individual, partnership, joint venture,
            -----------
corporation, trust, estate, unincorporated organization, government or any
department or agency thereof, or any other entity other than Amgen or Medarex.



     2.    IMMUNIZATION

     2.1   Antigen Availability.
           --------------------

          2.1.1  Antigen Selection and Replacement.
                 ---------------------------------

          (a)    At any time until [*****] the Effective Date, Amgen shall have
the right to notify Medarex of Amgen's interest in a particular Antigen. Any
such Antigen(s) with respect to which Medarex shall provide Amgen with notice of
availability thereof under Section 2.1.2(a) below (i) shall be placed on the
list attached hereto as Exhibit C, upon Amgen notifying Medarex that Amgen
desires to enter into a commercial license under Section 4.2 and paying to
Medarex the commercial license fee due pursuant to Section 5.4 with respect to
such Antigen, (ii) shall be placed on the list attached hereto as Exhibit C,
upon Amgen notifying Medarex that Amgen desires to enter into an Immunization
and paying to Medarex the immunization fee due pursuant to Section 5.2 with
respect to such Antigen or (iii) shall be placed on the list attached hereto as
Exhibit B on any of the following four (4) dates per year: March 1, June 1,
September 1 and December 1, upon Amgen notifying Medarex that Amgen desires to
take an option under Section 4.1, subject to Amgen paying to Medarex the
applicable Antigen Reservation Fee pursuant to Section 5.1.

          (b)    At any time upon Amgen exercising its option in accordance with
Section 4.1.1 with respect to an Antigen listed in Exhibit B, such Antigen shall
be removed from

***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       6
<PAGE>

Exhibit B upon such Antigen being placed in Exhibit C. In addition, once an
Antigen shall have been placed in Exhibit B, such Antigen may be removed by
Amgen from Exhibit B for any reason, on any of the following four (4) dates per
year: March 1, June 1, September 1 and December 1.

          (c)    Upon removal of an Antigen from Exhibit B, Amgen may designate
a replacement Antigen, such that at all times there may be up to and including
[*****] Antigen(s) listed in Exhibit B, subject to the availability of such
Antigen for inclusion in Exhibit B as described in Section 2.1.2(a) below and
Amgen paying to Medarex the applicable Antigen Reservation Fee pursuant to
Section 5.1, provided however, that [*****]. Any Antigen which is no longer
listed in Exhibit B shall no longer be an Antigen pursuant to this Agreement,
and Amgen shall have no further rights under this Agreement with respect
thereto, or rights to Product(s) directed against such Antigen, unless such
Antigen shall have been placed in Exhibit C and Amgen shall have option rights
under Section 4.1 and/or its exclusive license rights under Section 4.2 with
respect to Product(s) directed against such Antigen.

          (d)    The information contained within each notification provided by
Amgen pursuant to this Section 2.1.1, including Amgen's interest in such
Antigen, shall be considered the Confidential Information of Amgen.

          (e)    [*****].

          2.1.2  Notice of Availability: Notice Date.
                 -----------------------------------

          (a)    Within [*****] following receipt of notice from Amgen under
Section 2.1.1(a) that it is interested in a particular Antigen, Medarex shall
notify Amgen whether such Antigen shall be available for listing in Exhibit B or
C and possible immunization on behalf of Amgen. If such Antigen shall be
available for Immunization on behalf of Amgen, the Antigen shall [*****] such
notification, as appropriate, (i) be placed on the list in Exhibit C or (ii) be
placed on the list in Exhibit B for a period of [*****] as provided in Section
2.1.2(b) below, subject to [*****] Antigen limit described in Section 2.1.1(c)
and Amgen's payment of the Antigen reservation fee for such Antigen pursuant to
Section 5.1. [*****].

          (b)    With respect to each Antigen listed in Exhibit B, Amgen shall
have a period of [*****] from the date the Antigen is placed in Exhibit B (the
"Antigen Reservation Period") to exercise its right to request Medarex to
conduct an Immunization on behalf of Amgen or to acquire a license to such
Antigen. Amgen may extend the Antigen Reservation Period for a particular
Antigen listed in Exhibit B for [*****], subject to [*****] Antigen limit
described in

***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       7
<PAGE>

Section 2.1.1(c) and Amgen's payment of the Antigen reservation fee for such
Antigen pursuant to Section 5.1 for the applicable [*****] period; [*****].

          (c)    If an Antigen shall be available for immunization of Mice on
behalf of Amgen pursuant to Section 2.1.2(a), during the applicable Antigen
Reservation Period, if any, [*****].  Medarex hereby acknowledges that all
Antigen(s) set forth in Exhibit B as of the Effective Date are available for
immunization and Amgen shall have the rights set forth in Section 4.1 with
respect to all Product(s) directed against such Antigen(s).

          (d)    It is understood and agreed that the rights set forth in
Section 2.1.2(a) with respect to an Antigen may not be available for Amgen if
[*****].

          2.1.3  [*****].

          2.1.4  Extension.  Amgen may provide Medarex notice that it wishes to
                 ---------
have Medarex conduct an Immunization with any Antigen until [*****] the
Effective Date. Medarex shall have no obligation to initiate an Immunization
with any Antigen unless Amgen has provided notice under Section 2.2.1 before
[*****] the Effective Date. With the written agreement of the parties, Amgen may
select Antigens for Immunizations to be conducted after *****] the Effective
Date, on terms to be agreed by the parties.

     2.2  Immunizations.
          -------------

          2.2.1  Notice by Amgen.  From time-to-time during the term of this
                 ---------------
Agreement, Amgen may notify Medarex that it wishes to have Medarex use Mice to
prepare Antibody(ies) to an Antigen. At such time, the Antigen, if listed in
Appendix B, shall be removed from Appendix B and shall be listed in Appendix C.

          2.2.2  Medarex Activities.  Subject to the terms and conditions set
                 ------------------
forth herein, with respect to each Antigen listed in Exhibit C for which Amgen
shall provide Medarex notice under Section 2.2.1, during the term of the
applicable Immunization Period, Medarex shall perform its respective
obligations, including producing and characterizing Antibody(ies), in accordance
with a Research Plan, which shall be attached to and made part of Exhibit A
prior to each Immunization.

     2.3  Immunization Period.
          -------------------

          2.3.1  Initial [*****] Period.  With respect to each Immunization, the
                 ----------------------
initial term of the Immunization Period during which Medarex shall work pursuant
to Section 2.4 to prepare


***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       8
<PAGE>

Antibodies to an Antigen shall commence on the date an applicable Antigen shall
have been received by Medarex from Amgen pursuant to Section 2.5 and continue,
for up to [*****], until Medarex shall have generated and provided Amgen with
Antibodies meeting the criteria set forth in Section 2.4.

          2.3.2  Extension of Immunization Period. In the event Medarex shall
                 --------------------------------
be unable to generate Antibodies to an Antigen meeting the criteria set forth in
Section 2.4 within said [*****], the initial term of the Immunization Period may
be extended for such longer period (and for such additional fees) as the parties
may mutually agree.

     2.4  Diligent Efforts.  Medarex shall use diligent efforts to conduct each
          ----------------
Immunization in a professional manner and shall commit the personnel, facilities
and other resources necessary to perform each Immunization in order to prepare
Antibodies which have criteria as set forth in the applicable Research Plan;
provided however, Medarex does not warrant that any Immunization will result in
the preparation of an Antibody to such Antigen suitable for development as a
Product. If Medarex shall be unable to prepare an Antibody to a particular
Antigen during the [*****] Immunization Period that Amgen shall have been
providing funding pursuant to Section 5.2 to Medarex for such applicable
Immunization, then Medarex shall have no obligation to conduct any further work
with respect to such Antigen, and shall have no liability to Amgen hereunder for
such inability to produce such Antibody.

     2.5  Delivery of Antigens: License.  Amgen shall deliver to Medarex a
          -----------------------------
mutually agreed quantity of each Antigen in a form(s) as set forth in the
applicable Research Plan to immunize the Mice in connection with each
Immunization. Amgen hereby grants to Medarex a non-exclusive, non-transferable
license under all intellectual property owned or Controlled by Amgen to use each
Antigen(s) solely in accordance with the applicable Research Plan.

     2.6   [*****].

     2.7   [*****].

     2.8   [*****].

     2.9   [*****].

     2.10  [*****].

     3.   EVALUATION

***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       9
<PAGE>

     3.1  Delivery of Collaboration Technology.  Upon the preparation of an
          ------------------------------------
Antibody(ies) to an Antigen meeting the criteria set forth in the applicable
Research Plan [*****], Medarex shall promptly deliver to Amgen for evaluation by
Amgen (i) an agreed-upon number and quantity of such Antibody(ies), [*****].
Upon request by Amgen, Medarex shall provide reasonable assistance to Amgen in
the performance of Amgen's activities conducted with respect to the applicable
Evaluation.

     3.2  Evaluation License.  On an Antigen-by-Antigen basis, commencing on the
          ------------------
delivery to Amgen of the Antibody(ies) to such Antigen under Section 3.1[*****].

     3.3  Evaluation Period.
          -----------------

          3.3.1  Initial [*****] Period.  The initial term of the Evaluation
                 ----------------------
Period for a particular Antigen shall commence on the date that Amgen shall
receive from Medarex the Antibody(ies) to such Antigen [*****] and shall
terminate [*****].

          3.3.2  Extension of Evaluation Period.  Amgen shall have the option
                 ------------------------------
to extend the initial term of the Evaluation Period for an Antigen and the
corresponding Evaluation license under Section 3.2, for [*****], by providing
Medarex notice at least [*****] before the end of the initial term of the
Evaluation Period and concurrently paying to Medarex the extension fee due
pursuant to Section 5.2.

     4.   OPTION; RIGHT OF LAST REFUSAL; LICENSE RIGHTS

     4.1  Option for Commercial Licenses.  Amgen shall have an option to
          ------------------------------
obtain a commercial license with respect to Product(s) directed against such
Antigen in the Field of Use in all countries of the Territory, from the time an
Antigen shall be first listed in Exhibit B or C pursuant to Section
2.1.1(a)(iii) or (ii), respectively, for such time that the Antigen shall be
listed in either of Exhibit B or C (except Amgen shall not have such option (i)
upon termination of the respective Immunization Period without initiation of the
Evaluation Period with respect to such Antigen or without Amgen electing to
obtain a commercial license to develop and commercialize Products(s) directed
against such Antigen as set forth in Section 4.2) or (ii) upon termination of
the respective Evaluation Period with respect to such Antigen without Amgen
electing to obtain a commercial license to develop and commercialize Products(s)
directed against such Antigen as set forth in Section 4.2).

          4.1.1  Exercise.  At any time during the term of the option with
                 --------
respect to an Antigen as set forth in Section 4.1 above, Amgen may exercise its
option for such commercial license by providing notice to Medarex.

***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       10
<PAGE>

          4.1.2  License Fee. [*****] after providing notice to Medarex under
                 -----------
Section 4.1.1 with respect to an Antigen, Amgen shall pay to Medarex the
commercial license fee due pursuant to Section 5.4 with respect to such Antigen.

          4.1.3  [*****].

     4.2  Commercial License.
          ------------------

          4.2.1  Grant.  Upon a particular Antigen being placed on Exhibit C
                 -----
pursuant to Section 2.1.1(a)(i) or upon Amgen exercising its option under
Section 4.1 with respect to a particular Antigen, Medarex shall grant to Amgen
an exclusive (even as to Medarex) license, with the right to sublicense, under
the Licensed Technology to make, have made, import, have imported, use, offer
for sale and sell Product(s) directed against such Antigen in the Field of Use
in the Territory.

          4.2.2  Sublicenses. [*****] execution of any sublicense agreement
                 -----------
under Section 4.2.1, Amgen shall provide Medarex with at least the following
information with respect to each potential Sublicensee: (i) the identity of the
Sublicensee; (ii) a description of the Product(s), and the rights being granted
to the Sublicensee; and (iii) the territory in which the Product(s) will be
sold. Each sublicense granted by Amgen shall be consistent with all the terms
and conditions of this Agreement, and subordinate thereto, and Amgen shall
remain responsible to Medarex for the compliance of each such Sublicensee with
the financial and other obligations due under this Agreement.

          4.2.3  Acknowledgement.  It is understood and agreed that the
                 ---------------
foregoing license in Section 4.2.1 above shall not grant to Amgen any right to
make any transgenic animal or use any transgenic animal for any purpose.

     4.3  [*****].

     4.4  Retained Rights: No Further Rights.  Only the licenses granted
          ----------------------------------
pursuant to the express terms of this Agreement shall be of any legal force or
effect. [*****]

     4.5  [*****].

     4.6  [*****].


***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       11
<PAGE>

     5.   CONSIDERATION

     5.1  Antigen Reservation Fee.  For each Antigen for which Amgen shall
          -----------------------
believe it may desire to obtain an exclusive license to Product(s) directed
against such Antigen and shall desire to list in Exhibit B pursuant to Section
2.1.3, Amgen shall pay to Medarex an Antigen reservation fee of [*****] per
Antigen for each [*****] period (the "Antigen Reservation Period"). Such Antigen
reservation fee shall be paid to Medarex within [*****] of the date Medarex
shall notify Amgen under Section 2.1.2(a) that such Antigen is first listed in
Exhibit B. If Amgen shall desire to extend the Antigen Reservation Period for a
particular Antigen, and such Antigen is available for continued reservation, as
described in Section 2.1.2(b), then the Antigen Reservation Period for such
Antigen may be extended by Amgen for [*****] with notice to Medarex and payment
to Medarex of [*****], prior to the expiration of [*****].

     5.2  Immunization Fee.  During the initial [*****] day term of the
          ----------------
Immunization Period with respect to each Antigen, Amgen shall pay to Medarex an
immunization fee of [*****] per [*****] period. The first payment shall be due
within [*****] following the date of commencement of the Immunization Period in
accordance with Section 2.3.1 and, if appropriate, a second payment for a
[*****] period shall be due on or before the [*****]. The amount and timing of
any subsequent payments for any mutually-agreed-upon extensions of the initial
term of an Immunization Period pursuant to Section 2.3.2 shall be as mutually
agreed upon by the parties. Subject to Section 4.1.3, such payments shall be
non-refundable and non-creditable against other amounts due Medarex under this
Agreement.

     5.3  Evaluation Period Extension Fee.  If Amgen elects to extend the
          -------------------------------
initial term of an Evaluation Period for a particular Antigen pursuant to
Section 3.3.2, it shall pay to Medarex [*****] for a [*****] extension, which
amount shall be paid to Medarex no later than [*****] following the date of
commencement of the Evaluation Period in accordance with Section 3.3.1.

     5.4  Commercial License Fee.
          ----------------------

          5.4.1  Amount.  If Amgen shall exercise its option pursuant to
                 ------
Section 4.1 to acquire from Medarex a commercial license under Section 4.2 with
respect to Product(s) directed against an Antigen, within [*****] after Amgen's
notice to Medarex of its exercise of such option, Amgen shall pay to Medarex a
commercial license fee of [*****] per Antigen.

          5.4.2  Credit for Evaluation Period Extension Fee.  If Amgen shall
                 ------------------------------------------
have extended the initial term of the Evaluation Period for an Antigen beyond
the [*****] period pursuant to Section 3.3.2, and the commercial license for
such Antigen shall be obtained during the extension period of that Evaluation
license under Section 3.2, then the extension fees shall be


***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       12
<PAGE>

pro-rated based on the remainder of the Evaluation Period with respect to the
applicable Antigen and the pro rata amount of such extension fee shall be
credited against the commercial license fee.

     5.5  Milestone Payments.
          ------------------

          5.5.1  Milestone Payment(s).  Within thirty (30) days following the
                 --------------------
first occurrence of the relevant event(s) with respect to the first Product(s)
directed against an Antigen specified below ("Milestone"), Amgen shall pay to
Medarex the following one-time milestone payments ("Milestone Payment(s)"):

     [*****]

          5.5.2  [*****]

          5.5.3  [*****]

     5.6  Royalties.
          ---------

          5.6.1  Royalty on Net Sales.  In partial consideration for the
                 --------------------
commercial license, Amgen shall pay to Medarex a Royalty on annual Net Sales of
Product(s), on a Product-by-Product basis, as follows:

          [*****]

          5.6.2  Royalty Term.  The Royalties due pursuant to Section 5.6.1
                 ------------
shall be payable on a country-by-country and Product-by-Product basis [*****].

          5.6.3  Third Party Royalties.
                 ---------------------

          (a)    Medarex shall be responsible for payment to the Medical
Research Council ("MRC") of any royalties due the MRC pursuant to the MRC
Agreement; provided however, in the event that Amgen obtains a direct license
from the Medical Research Council with respect to the intellectual property
subject to the MRC License, all royalties paid to the MRC under such a license
with respect to Product(s) shall be fully creditable against Royalties due
Medarex under Section 5.6.1, on a Product-by-Product basis.

          (b)    Except with respect to the MRC Agreement under Section
5.6.3(a), Amgen shall be responsible for the payment of any royalties, license
fees and milestone and


***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       13
<PAGE>

other payments to a Third Party during the term of the Agreement under
agreement(s) for license rights to one or more patent claims which would be
infringed by the making, having made, using, offering to sell, selling and
importing of a Product(s) and/or other intellectual property necessary for such
activities.

          5.6.4  [*****].

          5.6.5  [*****].

          5.6.6  [*****].

          5.6.7  Cumulative Royalties.  The obligation to pay Royalties on the
                 --------------------
 Net Sales of a Product under this Section 5.6 shall be imposed only once with
respect to the same unit of said Product due under Section 5.6.1, regardless of
the number of claims within Licensed Patent Rights which would, but for this
Agreement, be infringed by the making, having made, using, offering to sell,
selling or importing of said Product.

          5.6.8  Paid-Up License.  Upon the expiration of Amgen's obligation
                 ---------------
pursuant to Section 5.6.1 to pay Royalties on Net Sales of a Product in a
country, Amgen shall have a fully paid-up, unrestricted, non-exclusive license
under Licensed Technology to make, have made, use, sell, offer to sell and
import said Product in the Field of Use in that country.

          5.6.9  [*****]

     5.7  [*****]

          5.7.1  [*****]

          5.7.2  [*****]

          5.7.3  [*****]

     6.   PAYMENTS

     6.1  Timing of Royalty Payments.  All Royalties on Net Sales due to Medarex
          --------------------------
shall be paid within [*****] after the last day of the calendar quarter in which
such Net Sales accrue.

     6.2  Payment Method.  All amounts due Medarex hereunder shall be paid in
          --------------
U.S. dollars by wire transfer in immediately available funds to an account
designated by Medarex.


***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       14
<PAGE>

Any payments or portions thereof due hereunder which shall not be paid on the
date such payments are due under this Agreement shall bear interest at a rate
equal to the lesser of the prime rate as reported by the Chase Manhattan Bank,
New York plus [*****] or of the maximum rate permitted by law, calculated on the
number of days such payment shall be delinquent compounded monthly. This Section
6.2 shall in no way limit any other remedies available to Medarex.

     6.3  Currency: Foreign Payments.  If any currency conversion shall be
          --------------------------
required in connection with the payment of any Royalties hereunder, such
currencies shall be converted into the United States Dollar equivalent, at the
average rate of exchange for the quarter (as calculated by averaging the rate of
exchange at the close of business of the last day of each month of said quarter)
as reported in International Financial Statistics (publisher, International
Monetary Fund) during the Royalty period of such Net Sales, or in the event
International Financial Statistics shall not be available then as reported in
The Wall Street Journal, for the currency of the country in which the sale shall
be made at the average rate of exchange during the Royalty period of such Net
Sales. If at any time legal restrictions or other reasons beyond Amgen's control
prevent the prompt remittance of any Royalties owed on Net Sales in any
jurisdiction, Amgen may notify Medarex and make such payments by depositing the
amount thereof in local currency in a bank account or other depository in such
country in the name of Medarex, and Amgen shall have no further obligations
under this Agreement with respect thereto.

     6.4  Taxes.  All Royalty amounts required to be paid to Medarex pursuant
          -----
to this Agreement may be paid with deduction for withholding or on account of
any taxes (other than taxes imposed on or measured by net income) or similar
governmental charge imposed by a jurisdiction other than the United States
("Withholding Taxes"). At Medarex's request, Amgen shall provide Medarex a
certificate evidencing payment of any Withholding Taxes hereunder sufficient to
enable Medarex to obtain the benefit of any applicable tax treaty.

     7.   REPORTS AND RECORDS

     7.1  Royalty Reports.  Beginning with the first calendar quarter in which
          ---------------
Royalties shall be due under Section 5.6.2 until the quarter during which there
shall be the expiration of Amgen's obligation to pay Royalties for any and all
Product(s) under Section 5.6.2, Amgen shall deliver to Medarex within [*****]
after the end of each calendar quarter in which a Product(s) shall be sold a
report setting forth the Net Sales of each Product(s) for such calendar quarter,
including the Product(s) sold in each country by Amgen and Sublicensee(s) and
the Net Sales thereof.  Such reports shall be the Confidential Information of
Amgen and will be maintained in confidence by Medarex in accordance with the
terms of Article 9 herein.


*****REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       15
<PAGE>

     7.2  Inspection of Books and Records.  Amgen and Sublicensee(s) shall
          -------------------------------
maintain accurate books and records which enable verification of the calculation
of Royalties payable hereunder. Amgen and Sublicensee(s) shall retain the books
and records for each quarterly period for [*****] after submission of the
corresponding report under Section 7.1 above. [*****] Medarex shall have the
option, upon [*****] prior notice to Amgen, to engage an independent accountant
from a "Big Five" accounting firm (selected by Medarex and approved by Amgen,
which approval will not be unreasonably withheld) to conduct a review or audit
of [*****] Any such inspection or audit shall be at Medarex's expense; however,
in the event an inspection reveals, as set forth in a final report, an
underpayment of [*****] or more in any quarter, Amgen shall pay the costs of the
inspection and [*****].

     8.   DEVELOPMENT AND COMMERCIALIZATION

     8.1  [*****]

     8.2  [*****]

     8.3  [*****]

     8.4  [*****]

     9.   CONFIDENTIALITY

     9.1  Confidential Information.  Except as expressly provided herein, the
          ------------------------
parties agree that the receiving party of any Confidential Information furnished
to it by the disclosing party pursuant to and as necessary for carrying out the
purposes of this Agreement shall keep such Confidential Information completely
confidential and shall not publish or otherwise disclose it (other than by Amgen
to Sublicensee(s)) and shall not use it for any purpose except for the purposes
contemplated by this Agreement, except to the extent that it can be established
by the receiving party by competent proof that such Confidential Information:

          9.1.1  was already known to the receiving party, other than under an
obligation of confidentiality, at the time of disclosure;

          9.1.2  was generally available to the public or otherwise part of the
public domain at the time of its disclosure to the receiving party;


*****REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       16
<PAGE>

          9.1.3  became generally available to the public or otherwise part of
the public domain after its disclosure, other than through any act or omission
of the receiving party in breach of this Agreement;

          9.1.4  was independently developed by the receiving party as
demonstrated by documented evidence prepared contemporaneously with such
independent development; or

          9.1.5  was subsequently lawfully disclosed to the receiving party by a
person other than a party hereto.

     9.2  Permitted Use and Disclosures.
          ------------------------------

          9.2.1  Each party may use or disclose Confidential Information
disclosed to it by the other party to the extent such use or disclosure shall be
required [*****] in complying with applicable governmental regulations or
otherwise submitting information to tax or other governmental authorities;
provided however, that if a party shall be required to make any such disclosure
of the other party's Confidential Information under this Section 9.2.1 it shall
give reasonable advance notice to the other party of such disclosure and shall
use reasonable efforts to secure confidential treatment (whether through
protective orders or otherwise) of such Confidential Information required to be
disclosed.

          9.2.2  Amgen (or Sublicensee(s)) may disclose the Confidential
Information of Medarex to the extent such disclosure shall be reasonably
necessary in (i) filing and prosecuting patent applications and maintaining
patents, [*****] and with respect to disclosures pursuant to Sections 9.2.2(i)
and 9.2.2(iv) above shall make reasonable efforts to disclose only information
required to be disclosed.

          9.2.3  [*****]

          9.2.4  Notwithstanding the above, in no event shall Amgen or
Sublicensee(s) disclose any Confidential Information of Medarex or Confidential
Information which is jointly developed by Medarex and Amgen to Abgenix, Inc. or
its Affiliates.

     9.3  Public Disclosure.  Except as otherwise required by law or regulation,
          -----------------
neither party shall issue a press release or make any other disclosure of the
existence of or the terms of this Agreement or any aspect of the research
conducted pursuant to this Agreement without the prior approval of such press
release or disclosure by the other party hereto.  Each party shall submit any
such press release or disclosure to the other party, and the receiving party
shall have [*****] to review and approve any such press release or disclosure,
which approval shall not be


*****REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       17
<PAGE>

unreasonably withheld. If the receiving party shall not respond within such
[*****] period, the press release or disclosure shall be deemed approved. In
addition if, in the reasonable opinion of a disclosing party's counsel, a public
disclosure shall be required by law, regulation or court order, including
without limitation, in a filing with the United States Securities and Exchange
Commission, the disclosing party shall provide copies of the disclosure
reasonably in advance of such filing or other disclosure for the nondisclosing
party's prior review and comment; the nondisclosing party shall provide its
comments, if any, on such announcement as soon as practicable.

     9.4  Confidential Terms.  Notwithstanding anything to the contrary herein,
          ------------------
(a) each party may disclose the terms of this Agreement or any aspect of the
research conducted pursuant to this Agreement in confidence under terms and
conditions at least as restrictive as set forth herein, on a need-to-know basis
to its legal and financial advisors to the extent such disclosure shall be
reasonably necessary in connection with such party's activities as expressly
permitted by this Agreement or [*****]. Notwithstanding the above, in no event
shall Amgen or Sublicensee(s) disclose any Confidential Information of Medarex
to Abgenix, Inc. or its Affiliates.

     9.5  [*****]

     10.  REPRESENTATIONS, WARRANTIES [*****]

     10.1 Medarex.  Medarex represents and warrants that as of the Effective
          -------
Date:

          10.1.1  it is a corporation duly organized, validly existing and in
good standing under the laws of the State of New Jersey, and it has the
corporate power and authority and the legal right to enter into this Agreement
and to perform its obligations hereunder;

          10.1.2  the execution, delivery and performance of this Agreement have
been duly authorized by all necessary corporate action on the part of Medarex,
and the person executing this Agreement on behalf of Medarex has been duly
authorized to do so by all requisite corporate actions;

          10.1.3  it is the sole and exclusive owner of all right, title and
interest in the Mice;

          10.1.4  it has the right to grant the rights and licenses granted
herein;

          [*****]


*****REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       18
<PAGE>

     10.2  Amgen.  Amgen represents and warrants that as of the Effective Date:
           -----

           10.2.1  it is a corporation duly organized, validly existing and in
good standing under the laws of the State of Delaware, and it has the corporate
power and authority and the legal right to enter into this Agreement and to
perform its obligations hereunder;

           10.2.2  the execution, delivery and performance of this Agreement
have been duly authorized by all necessary corporate action on the part of
Amgen, and the person executing this Agreement on behalf of Amgen has been duly
authorized to do so by all requisite corporate actions;

           [*****]

     10.4  Disclaimer of Warranties. [*****] ARE PROVIDED TO AMGEN "AS IS", AND
           ------------------------
EXCEPT AS OTHERWISE EXPRESSLY SET FORTH IN THIS AGREEMENT, MEDAREX MAKES NO
REPRESENTATIONS AND EXTENDS NO WARRANTIES OR CONDITIONS OF ANY KIND, EITHER
EXPRESS OR IMPLIED, WITH RESPECT TO THE MICE, PRODUCT(S) OR LICENSED TECHNOLOGY,
INCLUDING BUT NOT LIMITED TO, WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
PARTICULAR PURPOSE, VALIDITY OF THE PATENT RIGHTS LICENSED HEREUNDER, OR
NONINFRINGEMENT OF THE INTELLECTUAL PROPERTY RIGHTS OF THIRD PARTIES.

     10.5  Disclaimer.  Nothing in this Agreement is or shall be construed as:
           ----------

           10.5.1  [*****], a warranty or representation by Medarex as to the
validity or scope of any claim or patent within the Patent Rights;

           10.5.2  [*****], a warranty or representation that anything made,
used, sold, or otherwise disposed of under any license granted in this Agreement
is or will be free from infringement of any Patent Rights or other intellectual
property right of any Third Party;

           10.5.3  [*****], an obligation to bring or prosecute actions or suits
against Third Parties for infringement of any of the Medarex Patent Rights; or


*****REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       19
<PAGE>

           10.5.4  [*****], granting by implication, estoppel, or otherwise any
licenses or rights under patents or other rights of Medarex or Third Parties,
regardless of whether such patents or other rights are subordinate to any patent
within the Licensed Patent Rights.

     11.   INTELLECTUAL PROPERTY

     11.1  Ownership.
           ---------

           11.1.1  [*****]

     11.2  Prosecution of Patent Rights.
           ----------------------------

           11.2.1  [*****]

     11.3  [*****]

     11.4  [*****]

     11.5  [*****]

     12.   DISPUTE RESOLUTION

     [*****]

     13.   INDEMNIFICATION

     13.1  Medarex.  Medarex shall indemnify, defend and hold harmless Amgen
           -------
and its directors, officers and employees (each an "Amgen Indemnitee") from and
against any and all liabilities, damages, losses, costs or expenses (including
attorneys' and professional fees and other expenses of litigation and/or
arbitration) (each a "Liability") resulting from a claim, suit or proceeding
made or brought by a Third Party against an Amgen Indemnitee arising from or
occurring as a result of (i) any breach of the representations and warranties
set forth in Section 10.1, in each case, which breach was the proximate cause of
the relevant Liability, or (ii) the activities of Medarex hereunder, except, in
each case, under this Section 13.1(ii) to the extent caused by the negligence or
willful misconduct of Amgen.

     13.2  Amgen.  Amgen shall indemnify, defend and hold harmless Medarex and
           -----
its directors, officers and employees (each a "Medarex Indemnitee") from and
against any and all liabilities, damages, losses, costs or expenses (including
attorneys' and professional fees and


*****REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       20
<PAGE>

other expenses of litigation and/or arbitration) (each a "Liability") resulting
from a claim, suit or proceeding made or brought by a Third Party [*****]
against a Medarex Indemnitee, arising from or occurring as a result of (i) any
breach of the representations and warranties set forth in Section 10.2 in each
case, which breach was the proximate cause of the relevant Liability, (ii) the
practice by Amgen of any right granted herein, [*****] or to the extent caused
by the negligence or willful misconduct of Medarex, or (iii) any development,
testing, manufacture, importation, use, offer for sale, sale or other
distribution of any Product(s) by Amgen or Sublicensee(s) except, in each case,
under this Section 13.2(iii) to the extent caused by the negligence or willful
misconduct of Medarex.

     13.3  Procedure.  In the event that a Medarex Indemnitee or an Amgen
           ---------
Indemnitee intends to claim indemnification under this Article 13, it shall
promptly notify the other party (the "Indemnitor") in writing of such alleged
Liability. The Indemnitor shall have the sole right to control the defense and
settlement thereof. Each Indemnitee shall cooperate with the Indemnitor and its
legal representatives in the investigation of any action, claim or liability
covered by this Article 13. Each Indemnitee shall not, except at its own cost,
voluntarily make any payment or incur any expense with respect to any claim or
suit without the prior written consent of the Indemnitor, which the Indemnitor
shall not be required to give. [*****]

     13.4  [*****]

     14.   TERM AND TERMINATION

     14.1  Term.  The term of this Agreement shall commence on the Effective
           ----
Date. Unless earlier terminated as provided in this Article 14, this Agreement
shall continue in full force and effect until the later of [*****] on a country-
by-country basis, until there shall be no remaining Royalty payment obligations
for all Product(s) in a country. It is understood and agreed that Medarex shall
have no obligation to conduct any Immunizations with Antigens proposed by Amgen
after [*****] of the Effective Date.

     14.2  [*****]

     14.3  Effects of Termination of an Immunization or an Evaluation.  In the
           ----------------------------------------------------------
event that, with respect to an Antigen listed in Appendix C, Amgen terminates it
option or license rights under Section 14.2 or fails to exercise its option
under Section 4.1 within the specific time period to acquire the Commercial
License under Section 4.2:

          14.3.1  Medarex shall immediately cease use of and, within thirty (30)
days thereafter, to the extent not previously delivered to Amgen deliver to
Amgen copies of all


*****REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       21
<PAGE>

documents and other tangible embodiments disclosing or containing Amgen
Technology, Collaboration Technology and Joint Technology specifically and
solely relating to such Antigen (including Product(s)), other than with respect
to maintaining one (1) archival copy of documents disclosing Confidential
Information related thereto for its legal files, for the sole purpose of
determining its obligations hereunder. Further, Medarex shall at Amgen's request
destroy all Mice immunized with such Antigen, and all antibody(ies), antibody
cell(s) and genetic material(s) generated by Medarex pursuant to the respective
Immunization and shall provide Amgen with certification by an officer of Medarex
that all such materials have been destroyed or returned to Amgen, as
appropriate.

           14.3.2  The parties shall retain their respective ownership rights as
set forth in Section 11.1.

           14.3.3  [*****]

           14.3.4  [*****]

     14.4  Default.
           -------

           14.4.1  In the event any material representation or warranty made
hereunder by either party shall have been untrue in any material respect
("Representation Default") or upon any material breach or material default of a
material obligation of this Agreement by a party ("Performance Default"), the
party not in default ("Non-Defaulting Party") must first give the other party
("Defaulting Party") written notice thereof ("Notice of Default"), which notice
must state the nature of the Representation Default or Performance Default in
reasonable detail and request the Defaulting Party cure such default within
[*****], or within [*****] for non-payment. If the Defaulting Party shall
dispute the existence, extent or nature of any default set forth in a Notice of
Default, the parties shall use good faith efforts to resolve the dispute.

           14.4.2  [*****]

     14.5  Insolvency or Bankruptcy.
           ------------------------

           [*****]


     14.6  Effect of Termination.  Upon termination of this Agreement under this
           ---------------------
Article 14, the following rights and obligations shall apply:


*****REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       22
<PAGE>

          14.6.1  The parties shall retain their respective ownership rights as
set forth in Section 11.1.

          14.6.2  [*****].

          14.6.3  Medarex shall immediately cease use of and, within thirty (30)
days thereafter, to the extent not previously delivered to Amgen deliver to
Amgen copies of all documents and other tangible embodiments disclosing or
containing all Collaboration Technology and Joint Technology (including
Product(s) in each case that specifically and solely relates to the Antigen(s)
listed in Exhibit C) and of all Amgen Technology (including Product(s), other
than with respect to maintaining one (1) archival copy of documents disclosing
Confidential Information related thereto for its legal files, for the sole
purpose of determining its obligations hereunder. Further, Medarex shall at
Amgen's request destroy all Mice immunized with such Antigen(s) listed in
Exhibit C, and all antibody(ies), antibody cell(s) and genetic material(s)
generated by Medarex pursuant to this Agreement and shall provide Amgen with
certification by an officer of Medarex that all such materials shall have been
destroyed or returned to Amgen, as appropriate.

          14.6.4  [*****]

          14.6.5  Termination of any licenses under this Agreement or of this
Agreement for any reason shall not release either party hereto from any
liability which, at the time of such termination, shall have already accrued to
the other party or which shall be attributable to a period prior to such
termination nor preclude either party from pursuing any rights and remedies it
may have hereunder or by law or in equity with respect to any breach of this
Agreement. It is understood and agreed that, subject to Section 14.4, monetary
damages may not be a sufficient remedy for any breach of this Agreement and that
the non-breaching party may be entitled to injunctive relief as a remedy for any
such breach.

          14.6.6  Notwithstanding anything to the contrary in this Agreement,
except as provided in Article 13, neither party shall be liable for
consequential or incidental damages of any nature incurred by the other party
arising out of any Default under this Agreement.

          14.6.7  [*****]

          14.6.8  The option and license rights granted in Sections 3.2, 4.1 and
4.2 shall terminate upon any termination of this Agreement under this Article
14, and in such event Amgen and Sublicensee(s) shall immediately cease all
development and commercialization of Product(s), subject to Amgen's right to
dispose of Product(s) in accordance with Section 14.6.7.


***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       23
<PAGE>

           14.6.9   All Antigen Reservation Periods shall terminate
concurrently.

           14.6.10  Articles 6 (Payments), 7 (Reports and Records), 9
(Confidentiality), 11 (Intellectual Property), 12 (Dispute Resolution), 13
(Indemnification) and 15 (Miscellaneous) of this Agreement shall survive
termination of this Agreement for any reason [*****].

     15.   MISCELLANEOUS

     15.1  Governing Law.  This Agreement shall be governed by and interpreted
           -------------
 in accordance with the substantive laws of the State of Delaware, without
reference to the conflicts of law principles thereof. [*****]

     15.2  Independent Contractors.  The relationship of the parties hereto
           -----------------------
shall be that of independent contractors. The parties hereto shall not be deemed
to be agents, employer-employee, partners or joint venturers of the other for
any purpose as a result of this Agreement or of the transactions contemplated
thereby. No party shall incur any debts or make any commitments for the other
party except to the extent, if at all, specifically provided herein.

     15.3  Assignment.  This Agreement shall not be assignable by either party
           ----------
to any Third Party hereto without the written consent of the other party which
consent shall not be unreasonably withheld; except a party may assign this
Agreement, without such consent, to an entity that shall acquire all or
substantially all of its business or assets to which this Agreement pertains,
whether by merger, consolidation, reorganization, acquisition, sale or
otherwise. This Agreement shall be binding upon and inure to the benefit of the
parties and their successors and assigns, and the name of a party appearing
herein shall be deemed to include the names of such party's successors and
permitted assigns to the extent necessary to carry out the intent of this
Agreement. Any assignment not in accordance with this Section 15.3 shall be
void.

     15.4  Notices.  All notices, requests, approval or consent, and other
           -------
communications hereunder shall be deemed to have been sufficiently given if in
writing and shall be personally delivered or sent by telecopy, telegram or other
electronic facsimile transmission (receipt verified), or by registered or
certified mail (return receipt requested), postage prepaid, commercial overnight
courier, in each case to the respective address specified below, or such other
address as may be specified in writing to the other party hereto.

If  to Medarex:     Medarex, Inc.


***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       24
<PAGE>

               1545 Route 22 East
               Annandale, New Jersey  08801
               Attn:  President

If to Amgen:   Amgen Inc.
               One Amgen Center Drive
               Thousand Oaks, California  91320-1799
               Attn:  Lawrence M. Souza, Ph.D.
                      Senior Vice President, Research
               cc:    Secretary and General Counsel

     15.5  Force Majeure.  If the performance of any part of this Agreement by
           -------------
either party or of any obligation under this Agreement (other than for the
payment of money) shall be prevented, restricted, interfered with or delayed by,
for example, war, strike, fire, Act of God, earthquake, flood, lockout, embargo,
governmental acts or orders or restrictions, or failure of suppliers, neither
party shall lose any rights hereunder or be liable to the other party for
damages or losses (except for payment obligations) on account of failure of
timely performance by the nonperforming party occasioned where failure to timely
perform shall be beyond the reasonable control and not caused by the negligence,
intentional conduct or misconduct of the nonperforming party and the
nonperforming party shall have given written notice to the other party and shall
have exerted all reasonable efforts to avoid or remedy such force majeure and
shall continue performance with the utmost dispatch whenever such causes shall
be removed; provided however, that in no event shall a party be required to
settle any labor dispute or disturbance. When such circumstances shall arise,
the parties shall discuss what, if any, modification of the terms of this
Agreement may be required in order to arrive at an equitable solution.

     15.6  Advice of Counsel.  Medarex and Amgen have each consulted counsel of
           -----------------
 their choice regarding this Agreement, and each acknowledges and agrees that
this Agreement shall not be deemed to have been drafted by one party or another
and shall be construed accordingly.

     15.7  Compliance with Laws.  Each party shall furnish to the other party
           --------------------
any information reasonably requested by that party to enable that party to
comply with the requirements of any U.S. or foreign, federal, state and/or
government agency.

     15.8  Further Assurances.  From time to time, a party shall at the
           ------------------
request of the other party hereto (i) deliver to the requesting party any
records, data or other documents consistent with the provisions of this
Agreement, (ii) execute, and deliver or cause to be delivered, all such
consents, documents or further instruments of transfer or license, and (iii)
take or cause to be taken all such actions as may be reasonably deemed necessary
by the parties in order for the requesting party to obtain the full benefits of
this Agreement and the transactions contemplated hereby.


                                       25
<PAGE>

     15.9   Export Controls.  Each party agrees that it will take all actions
            ---------------
necessary to insure its compliance with all U.S. laws, regulations, orders or
other restrictions on exports and Amgen further agrees that it will not sell,
license or reexport, directly or indirectly, the Product(s) to any person or
entity for sale in any country or territory if, to the knowledge of Amgen based
upon reasonable inquiry, such sale would cause the parties to be in violation of
any such laws or regulations now or hereafter in effect.

     15.10  Severability.  In the event that any provision of this Agreement is
            ------------
determined to be illegal, invalid or unenforceable by a court of competent
jurisdiction, the parties shall in good faith negotiate a substitute clause for
any provision declared illegal, invalid or unenforceable, which shall most
nearly approximate the intent of the parties in entering this Agreement.  If
unreformable, such provision shall be divisible and deleted in such jurisdiction
so long as the parties shall still able to realize the principal benefits
bargained for in this Agreement.

     15.11  Waiver.  It is agreed that no waiver by either party hereto of any
            ------
breach or default of any of the covenants or agreements herein set forth shall
be deemed a waiver as to any subsequent and/or similar breach or default.

     15.12  Complete Agreement.  This Agreement, with its Exhibits, constitutes
            ------------------
the entire agreement, both written and oral, between the parties with respect to
the subject matter hereof, and all prior negotiations, correspondence,
representations, understandings and agreements respecting the subject matter
hereof, either written or oral, expressed or implied, are merged and canceled
and are null and void and of no effect. No amendment or change hereof or
addition hereto shall be effective or binding on either of the parties hereto
unless reduced to writing and duly executed an behalf of both parties. The
Materials Transfer Agreement dated April 22, 1999 (Amgen Reference No.
19992424), as amended effective May 31, 1999 (Amgen Reference No. 19992424-001)
is hereby superseded, provided that all Confidential Information and/or
materials disclosed, transferred, characterized, generated, derived, or
identified or the like under the Materials Transfer Agreement shall be treated
as if disclosed, transferred, characterized, generated, derived, or identified
or the like, and be subject to the terms of, this Agreement.

     15.13  Use of Name.  Except as required by law or in required filings with
            -----------
government entities, neither party shall use the name or trademarks of the other
party or the names of any employees thereof, without the prior written consent
of such other party.

     15.14  Headings.  The captions to the several Articles and Sections hereof
            --------
are included merely for convenience of reference only and shall not affect its
meaning nor the interpretation of any of the provisions of this Agreement.

     15.15  Counterparts.  This Agreement may be executed in two counterparts,
            ------------
each of which shall be deemed an original and which together shall constitute
one instrument.

                                       26
<PAGE>

     15.16  Exhibits.  All Exhibits referenced in and attached hereto are
            --------
incorporated herein by reference. In case of any discrepancies between language
incorporated from the Exhibits and the terms of the Articles and Sections
herein, the terms of the Articles and Sections shall prevail.

     IN WITNESS WHEREOF, Medarex, Inc., GenPharm International, Inc. and Amgen
have executed this Agreement by their respective duly authorized
representatives.

AMGEN INC.


By: /s/ Lawrence M. Souza
   -----------------------------------
Name:  Lawrence M. Souza, Ph.D.
Title: Senior Vice President, Research

MEDAREX, INC.                                 GENPHARM INTERNATIONAL, INC.


By: /s/ Michael Appelbaum                     By: /s/ Michael Applebaum
    ----------------------------------           -------------------------------
Name:  Michael Appelbaum                      Name:  Michael Appelbaum
Title: Executive Vice President               Title: President

                                       27
<PAGE>

EXHIBIT A
RESEARCH PLAN
-------------


[*****]



***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       28
<PAGE>

EXHIBIT B
ROLLING LIST OF ANTIGENS
------------------------

                                       29
<PAGE>

EXHIBIT C
OPTIONED OR LICENSED ANTIGENS
-----------------------------


[*****]


***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       30
<PAGE>

EXHIBIT D


[*****]


***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       31
<PAGE>

EXHIBIT E

[*****]


***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       32