Document:

<PAGE>

                           ONYX PHARMACEUTICALS, INC.

                                       AND

                             WARNER-LAMBERT COMPANY

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

                        STOCK PUT AND PURCHASE AGREEMENT

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

                                OCTOBER 13, 1999

<PAGE>

                           ONYX PHARMACEUTICALS, INC.
                                       AND
                             WARNER-LAMBERT COMPANY

                        STOCK PUT AND PURCHASE AGREEMENT

       THIS STOCK PUT AND PURCHASE AGREEMENT (the "Agreement") is made and
entered into on October 13, 1999, to be effective as of September 1, 1999, by
and between ONYX PHARMACEUTICALS, INC., a Delaware Corporation with its
principal office at 3031 Research Drive, Richmond, California 94806 (the
"Company"), and WARNER-LAMBERT COMPANY, a Delaware Corporation with its
principal office at 201 Tabor Road, Morris Plains, New Jersey 07950 (the
"Purchaser").

                                    RECITALS

       WHEREAS, the Company and Purchaser have entered into that certain
Collaboration Agreement of even date herewith (the "Collaboration Agreement");
and

       WHEREAS, in connection with the Collaboration Agreement, the Purchaser
desires to grant to the Company and the Company desires to receive from the
Purchaser certain options to cause the Purchaser to purchase shares of common
stock of the Company, on the terms and subject to the conditions set forth in
this Agreement.

       NOW, THEREFORE, in consideration of the foregoing recitals and the mutual
covenants and agreements contained herein, the parties hereto, intending to be
legally bound, do hereby agree as follows:

       1.    PUT RIGHTS OF THE COMPANY.

             1.1   FIRST AND SECOND PUT RIGHTS. At any time during the calendar
year 2000, subject to the terms of this Agreement, the Company shall have the
option ("First Put Right") to sell to the Purchaser, and cause the Purchaser to
purchase, no less than all the First Put Shares (as hereinafter defined). At any
time during the calendar year 2001, subject to the terms of this Agreement, the
Company shall have the option ("Second Put Right") to sell to the Purchaser, and
cause the Purchaser to purchase, no less than all the Second Put Shares (as
hereinafter defined). The First Put Right and Second Put Right may each be
referred herein individually as a "Put Right", or collectively as the "Put
Rights."

            1.2    PUT NOTIFICATION DATE; PUT DATE. The Company shall provide
written notice to the Purchaser of its election to exercise a Put Right, which
notice shall specify: (a) the date of notice of exercise of the Put Right (the
"Put Notification Date"); and (b) the date on which the Purchaser shall purchase
the Put Shares (as defined below) from the Company (the "Put Date"). The Put
Date in each case shall be the tenth business day after the Purchaser receives
such notice, subject to Section 5.2 and 5.3.

                                   1

<PAGE>

            1.3    FIRST PUT SHARES. The "First Put Shares" shall be defined as
that number of shares of common stock of the Company as determined by dividing
five million dollars ($5,000,000) by the Put Price Per Share applicable to the
exercise of the First Put Right, with any fractional share being rounded up to
the next whole number.

            1.4    SECOND PUT SHARES. The "Second Put Shares" shall be defined
as that number of shares of common stock of the Company as determined by
dividing five million dollars ($5,000,000) by the Put Price Per Share applicable
to the exercise of the Second Put Right, with any fractional share being rounded
up to the next whole number.

            1.5    PUT PRICE PER SHARE. The "Put Price Per Share", with respect
to exercise of a particular Put Right, shall be defined as the arithmetic
average of the closing sales prices of the common stock of the Company, as
reported by the NASDAQ, for the twenty (20) trading days immediately prior to
(but not including) the second business day before the Put Date applicable to
such exercise of such Put Right in which such shares of common stock are traded.
If there are any days in which NASDAQ is open but the common stock of the
Company does not trade, such days shall be omitted from the calculation and
additional days shall be added to the time period as necessary to establish a
20-trading day average price.

            1.6    SALE OF THE PUT SHARES. Subject to the terms and conditions
hereof, upon exercise by the Company of the First Put Right, the Company hereby
agrees to issue and sell to the Purchaser, and the Purchaser hereby agrees to
purchase from the Company, the First Put Shares for an aggregate purchase price
of five million dollars ($5,000,000). Subject to the terms and conditions
hereof, upon exercise by the Company of the Second Put Right, the Company hereby
agrees to issue and sell to the Purchaser, and the Purchaser hereby agrees to
purchase from the Company, the Second Put Shares for an aggregate purchase price
of five million dollars ($5,000,000).

       2.   CLOSING DATE; DELIVERY.

            2.1    FIRST CLOSING; FIRST CLOSING DATE. The closing of the sale
and purchase of the First Put Shares under this Agreement (the "First Closing")
shall be held at 9:00 a.m. (Pacific Time) on the first Put Date, at the offices
of Cooley Godward LLP, Five Palo Alto Square, 3000 El Camino Real, Palo Alto,
California, or at such other time and place as is provided for in Section 5.2 or
otherwise as the Company and the Purchaser may agree (the "First Closing Date").

            2.2    SECOND CLOSING; SECOND CLOSING DATE. The closing of the sale
and purchase of the Second Put Shares under this Agreement (the "Second
Closing") shall be at 9:00 a.m. (Pacific Time) on the second Put Date, at the
offices of Cooley Godward LLP, Five Palo Alto Square, 3000 El Camino Real, Palo
Alto, California, or at such other time and place as is provided for in Section
5.2 or otherwise as the Company and the Purchaser may agree (the "Second Closing
Date").

            2.3    DELIVERY. At the First Closing and the Second Closing,
subject to the terms and conditions hereof, the Company will deliver to the
Purchaser a stock certificate, issued in the name of the Purchaser and bearing
the legends referred to in Section 4.3, representing,

                                   2

<PAGE>

respectively, the First Put Shares and the Second Put Shares, dated as of the
First Closing and the Second Closing, respectively, against payment of the
purchase price therefor by wire transfer, unless other means of payment shall
have been agreed upon by the Purchaser and the Company.

       3.   REPRESENTATIONS AND WARRANTIES OF THE COMPANY. The Company hereby
represents and warrants to the Purchaser as follows:

            3.1    AUTHORIZATION. All corporate action on the part of the
Company, its officers, directors and stockholders necessary for the
authorization, execution and delivery of this Agreement has been taken, and,
with respect to the performance of the Company's obligations hereunder, will
have been taken at the time of the applicable Closing. The Company has the
requisite corporate power to enter into this Agreement and carry out and perform
its obligations under the terms of this Agreement, and this Agreement
constitutes a legally binding, valid obligation of the Company, enforceable in
accordance with its terms. At the First Closing and the Second Closing, the
Company will have the requisite corporate power to sell the shares of common
stock to be sold at each such closing.

            3.2    NO CONFLICT WITH OTHER INSTRUMENTS. The execution, delivery
and performance of this Agreement will not result in any violation of, or be in
conflict with, the Certificate of Incorporation or the Bylaws of the Company or
any provision of any judgment, decree or order to which the Company is a party
or by which it is bound, or any material statute, rule or governmental
regulation applicable to the Company, and will not constitute a material default
of any material contract, obligation or commitment to which the Company is a
party or by which it is bound.

            3.3    LITIGATION. There is no bona fide action, proceeding or
investigation pending or, to the knowledge of the Company, overtly threatened,
brought or threatened by a third party unrelated to the Purchaser, that is not
initiated, encouraged or supported by the Purchaser and that seeks to prohibit
the consummation of the transactions contemplated by this Agreement, nor has any
such third party asserted in a writing delivered to the Company that a basis for
such an action, proceeding or investigation exists.

            3.4    ORGANIZATION, GOOD STANDING AND QUALIFICATION. The Company is
a corporation duly organized, validly existing and in good standing under the
laws of the State of Delaware and has all requisite corporate power and
authority to carry on its business as now conducted and as proposed to be
conducted. The Company is duly qualified to transact business and is in good
standing in each jurisdiction in which the failure so to qualify would have a
material adverse effect on its business or properties.

            3.5    VALID ISSUANCE OF SHARES. The First Put Shares and the Second
Put Shares which will be purchased by Purchaser hereunder, when issued, sold and
delivered in accordance with the terms hereof for the consideration expressed
herein, will be duly and validly authorized and issued, fully paid and
nonassessable and, based in part upon the representations of Purchaser in this
Agreement, will be issued in compliance with all applicable federal and state
securities laws.

                               3

<PAGE>

            3.6    GOVERNMENTAL CONSENTS. No consent, approval, order or
authorization of, or registration, qualification, designation, declaration or
filing with, any federal, state, local or provincial governmental authority on
the part of the Company is required in connection with the consummation of the
transactions contemplated by this Agreement, except for notices required or
permitted to be filed with certain state and federal securities commissions,
which notices will be filed on a timely basis, and otherwise as provided in
Section 5.1.

            3.7    SHARES AVAILABLE. The Company shall have available at the
First Closing Date and the Second Closing Date, sufficient authorized but
unissued shares of its Common Stock to issue and sell to the Purchaser all of
the Put Shares.

            3.8    FULL DISCLOSURE. The representations and warranties of the
Company contained in this Agreement do not contain any untrue statement of a
material fact or omit any material fact necessary to make the statements
contained herein in view of the circumstances under which they are made not
misleading.

            3.9    BROKERS AND FINDERS. The Company has not retained any
investment banker, broker or finder in connection with the transaction
contemplated by this Agreement.

       4.   REPRESENTATIONS AND WARRANTIES OF THE PURCHASER.

            The Purchaser hereby represents and warrants to the Company as
follows:

            4.1    LEGAL POWER. The Purchaser has the requisite corporate power
to enter into this Agreement, to carry out and perform its obligations under the
terms of this Agreement and, at the First Closing and the Second Closing, will
have the requisite corporate power to purchase, respectively, the First Put
Shares and the Second Put Shares to be purchased at each such Closing.

            4.2    DUE EXECUTION. This Agreement has been duly authorized,
executed and delivered by the Purchaser, and, upon due execution and delivery by
the Company, this Agreement will be a valid and binding agreement of the
Purchaser, except as enforceability may be limited by bankruptcy, insolvency,
reorganization, moratorium or similar laws affecting creditors' rights generally
or by equitable principles.

       4.3  INVESTMENT REPRESENTATIONS.

            (a)    Purchaser is acquiring the First Put Shares and the Second
Put Shares for its own account, not as nominee or agent, for investment and not
with a view to, or for resale in connection with, any distribution or public
offering thereof within the meaning of the Securities Act of 1933, as amended
(the "Securities Act").

            (b)    Purchaser understands that:

                   (i)   the First Put Shares and the Second Put Shares have not
been registered under the Securities Act by reason of a specific exemption
therefrom, that such securities must be held by it indefinitely, absent such
exemption, and that Purchaser must,

                                  4

<PAGE>

therefore, bear the economic risk of such investment indefinitely, unless a
subsequent disposition thereof is registered under the Securities Act or is
exempt from such registration;

                   (ii)   each certificate representing the First Put Shares and
the Second Put Shares will be endorsed with the following legends:

                          (A)  THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN
REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE "ACT"), OR UNDER
THE SECURITIES LAWS OF CERTAIN STATES. THESE SECURITIES ARE SUBJECT TO
RESTRICTIONS ON TRANSFERABILITY AND RESALE AND MAY NOT BE TRANSFERRED OR RESOLD
EXCEPT AS PERMITTED UNDER THE ACT AND THE APPLICABLE STATE SECURITIES LAWS,
PURSUANT TO REGISTRATION OR EXEMPTION THEREFROM. THE ISSUER OF THESE SECURITIES
MAY REQUIRE AN OPINION OF COUNSEL IN FORM AND SUBSTANCE SATISFACTORY TO THE
ISSUER TO THE EFFECT THAT ANY PROPOSED TRANSFER OR RESALE IS IN COMPLIANCE WITH
THE ACT AND ANY APPLICABLE STATE SECURITIES LAWS; and

                          (B)  Any legend required to be placed thereon by the
applicable state securities laws; and

                   (iii)  the Company will instruct any transfer agent not to
register the transfer of either the First Put Shares (or any portion thereof) or
the Second Put Shares (or any portion thereof) unless the conditions specified
in the foregoing legends are satisfied, until such time as a transfer is made,
pursuant to the terms of this Agreement, and in compliance with Rule 144 of the
General Rules and Regulations prescribed by the Securities and Exchange
Commission pursuant to the Act ("Rule 144") or pursuant to a registration
statement or, if the opinion of counsel referred to above is to the further
effect that such legend is not required in order to establish compliance with
any provisions of the Act or this Agreement.

            (c)    The Purchaser has been furnished with such materials and has
been given access to such information relating to the Company as its qualified
representative has requested, and the Purchaser has been afforded the
opportunity to ask questions regarding the Company, the First Put Shares and the
Second Put Shares, all as it has found necessary to make an informed investment
decision.

            (d)    The Purchaser is an "accredited investor" as such term is
defined in Rule 501 of the General Rules and Regulations prescribed by the
Securities and Exchange Commission pursuant to the Act, and the Purchaser was
not formed for the specific purpose of acquiring the First Put Shares or the
Second Put Shares.

       4.4  STANDSTILL COVENANT. Purchaser agrees that, during the period
commencing on the date hereof and ending on May 14, 2003, neither Purchaser
nor any of its affiliates shall acquire any additional shares of capital
stock or other securities of the Company other than as provided in Article 1
and this Section 4.4 of this Agreement without the prior written consent of
the Board of Directors of the Company (the "Board") in the Board's discretion
but subject to the Board's fiduciary duties, provided that in the event that
any person, entity or

                                 5

<PAGE>

group shall acquire or make a bona fide offer to acquire, in one or more
transactions by tender or otherwise, shares of the Company equal to or exceeding
the percentage of the fully diluted voting power then held by the Purchaser,
then the Purchaser shall be allowed to acquire additional shares of the Company
free of the aforesaid restrictions in response to such acquisition or offer but
only for so long as such offer shall remain outstanding. Nothing in this Section
4.4 shall prohibit the acquisition or disposition of shares for investment
purposes only in the open market in the ordinary course by any pension fund or
trust for the benefit of employees of the Purchaser or its affiliates.

       5.   HART-SCOTT-RODINO FILINGS;  CLOSING CONDITIONS

            5.1    HART-SCOTT-RODINO. Each notice of exercise of Put Right
delivered by the Company pursuant to Section 1.2 shall also include a statement
as to whether a filing will be required under the Hart-Scott-Rodino Antitrust
Improvements Act of 1976, as amended, or any other governmental body, in order
to close the stock purchase contemplated by such notice of exercise, and the
basis for such conclusion. If such a filing is required, each of the Company and
the Purchaser shall use its best efforts to promptly effect such filing and
satisfy the requirements of such act. The Company and the Purchaser shall share
equally any filing fees required to obtain such consent.

            5.2    CLOSING DATE. If a Hart-Scott-Rodino filing is required, the
closing shall occur on a date specified by the Company by notice to the
Purchaser provided after the expiration of any applicable waiting period or
other satisfaction of the Hart-Scott-Rodino requirements, which date shall be
not less than five business days after such expiration or other satisfaction.

            5.3    CONDITIONS TO CLOSING. The closing of any purchaser and sale
of common stock under this Agreement shall be subject to the satisfaction of the
following conditions. Each of paragraphs (a) through (e), (g) and (h) shall be a
condition to the obligations of Purchaser at each of the first and the second
Put Dates, and each of paragraphs (b), (c), (d), (f) and (h) shall be a
condition to the obligations of the Company at each of the first and the second
Put Dates. Each of the Purchaser and the Company may waive any condition
applicable to its obligations hereunder. In the event that one or more
conditions in this Section 5.3 are not satisfied or waived as of the applicable
Put Date, then the applicable Put Date and Closing shall be delayed until such
time as all conditions under this Section 5.3 are satisfied or waived (and in
which case the Put Price Per Share shall be calculated as if the Put Date were
the tenth business day after the Purchaser receives such original notice of the
put pursuant to Section 1.2), unless, with respect to unsatisfied and unwaived
conditions to the obligations of the Purchaser, the Company otherwise informs
Purchaser in writing.

                   (a)    COMPANY'S REPRESENTATIONS AND WARRANTIES;
COLLABORATION AGREEMENT. The representations and warranties of the Company
contained in Article 3 shall be true on and as of the Put Date with the same
effect as if made on and as of the Put Date, and the Company shall not be in
material breach under that certain Collaboration Agreement between the parties
of even date herewith and such Collaboration Agreement shall not have been
terminated in its entirety by Purchaser pursuant to the terms thereof.

                                 6

<PAGE>

                   (b)    BLUE SKY COMPLIANCE. The Company shall have complied
with and be effective under all state securities or Blue Sky laws applicable to
the offer and sale of the Put Shares then being sold.

                   (c)    HART-SCOTT-RODINO. Any requirements of the Hart-Scott-
Rodino Antitrust Improvements Act of 1976, as amended, shall have been
satisfied.

                   (d)    COURT ORDERS. There shall not be in effect any
injunction or restraining order issued by any court of competent jurisdiction in
any action or proceeding against the consummation of the sale and purchase of
the Put Shares.

                   (e)    PROCEEDINGS SATISFACTORY; COMPLIANCE CERTIFICATE. All
corporate and legal proceedings taken by the Company in connection with the sale
and issuance of the Put Shares and all documents and papers relating to such
transaction shall be satisfactory to the Purchaser and its counsel, in the
reasonable judgment of the Purchaser and its counsel, and the Purchaser shall
have received all such counterpart original and certified or other copies of
such documents as it may reasonably request. The Company shall have delivered to
the Purchaser a certificate, dated as of the closing date, signed by the
Company's President, certifying that the conditions set forth in Section 5.3(a)
through (e), (g) and (h) have been satisfied.

                   (f)    PURCHASER'S REPRESENTATIONS AND WARRANTIES;
PERFORMANCE. The representations and warranties of the Purchaser contained in
Article 4 shall be true on and as of the applicable Put Date with the same
effect as through said representations and warranties had been made on and as of
such Put Date. The Company shall have performed or fulfilled in all material
respects, all conditions contained in this Section 5.3 and all agreements and
obligations contained in this Agreement required to be performed or fulfilled by
the Company before each of the First Closing and Second Closing, as the case may
be.

                   (g)    OPINION OF COUNSEL. Purchaser shall have received an
opinion of Cooley Godward LLP, counsel to the Company, dated the applicable Put
Date, in substantially the form of Exhibit A attached hereto.

                   (h)    THIRD PARTY CONSENTS AND APPROVALS. The Company shall
have obtained all necessary third party consents and approvals necessary for the
consummation of the transactions contemplated hereby.

       6.   REGISTRATION RIGHTS.

            6.1    REGISTRATION. The terms "Register", "Registered" and
"Registration" refer to a registration effected by preparing and filing a
registration statement in compliance with the Securities Act of 1933, as
amended, or any similar federal statutes, and the rules and regulations of the
Commission thereunder ("Registration Statement"), and the declaration or
ordering of the effectiveness of such Registration Statement.

            6.2    SUBORDINATION. In accordance with Article 12 of the Amended
and Restated Information and Registration Rights Agreement dated as of May 14,
1994, as amended, by and among the Company and certain investors of the Company
described therein, the

                                   7

<PAGE>

Registration rights of Purchaser under this Article 6 shall be subordinate to
the Registration rights granted to the Holders (as defined in the Rights
Agreement) pursuant the Rights Agreement.

            6.3    NOTICE OF REGISTRATION AND INCLUSION OF SHARES. Subject to
the terms of this Agreement, in the event the Company decides to Register any of
its Common Stock (either for its own account or the account of a security holder
or holders exercising their respective demand Registration rights) on a form
that would be suitable for a Registration involving solely the Put Shares, the
Company will (i) promptly give the Purchaser written notice thereof (which shall
include a list of the jurisdictions in which the Company intends to attempt to
qualify such securities under the applicable Blue Sky or other state securities
laws) and (ii) include in such Registration (and any related qualification under
Blue Sky laws or other compliance), and in any underwriting involved therein
(subject to Section 6.4), all the Put Shares that the Purchaser has purchased
pursuant to this Agreement and any Common Stock issued pursuant to stock splits,
stock dividends and similar distributions with respect to the Put Shares
(collectively the "Warner Registrable Shares") specified in a written request
delivered to the Company by the Purchaser within twenty (20) days after delivery
of such written notice from the Company.

       6.4  UNDERWRITING IN REGISTRATION.

            (a)    NOTICE OF UNDERWRITING IN REGISTRATION. If the Registration
of which the Company gives notice under Section 6.3 is for a Registered public
offering involving an underwriting, the Company shall so advise the Purchaser as
a part of the written notice given pursuant to Section 6.3. In such event, and
notwithstanding Section 6.3, the right of the Purchaser to Registration shall be
conditioned upon such underwriting and the inclusion of the Warner Registrable
Shares in such underwriting to the extent provided in this Section 6.4. If
Purchaser proposes to distribute the Warner Registrable Shares through such
underwriting, it shall (together with the Company and any other holders
distributing their securities through such underwriting) enter into an
underwriting agreement with the representative of the underwriter or
underwriters selected for such underwriting for such offering ("Underwriter's
Representative"). The Purchaser shall have no right to participate in the
selection of the underwriters for an offering pursuant to this Article 6.

            (b)    MARKETING LIMITATION IN REGISTRATION. In the event the
Underwriter's Representative advises the Purchaser in writing that market
factors (including, without limitation, the aggregate number of shares of Common
Stock requested to be Registered, the general condition of the market, and the
status of the persons proposing to sell securities pursuant to the Registration)
require a limitation of the number of shares to be underwritten, the
Underwriter's Representative may exclude some or all Warner Registrable Shares
from such Registration and underwriting. In such event, the Underwriter's
Representative shall so advise the Purchaser and all holders of securities
requested and otherwise entitled to be included in such Registration, and the
number of shares that may be included in the underwriting shall be allocated,
(i) in the event there are no other securities which would have priority over
the Warner Registrable Shares in such allocation, first, to the Company; and
second, to the Purchaser and any other shareholders of the Company's securities
then having registration rights with respect to the Company's securities on a
pro rata basis based on the total number of Warner Registrable Shares held by
the Purchaser and the total number of registrable shares held by such other

                                 8

<PAGE>

shareholders, or (ii) in the event there are other securities that would have
priority over the Warner Registrable Shares in such allocation, first, to the
Company and the holders of such other shares; and second, to the Purchaser and
any other shareholders of the Company's securities then having registration
rights with respect to the Company's securities on a pro rata basis based on the
total number of Warner Registrable Shares held by the Purchaser and the total
number of registrable shares held by such other shareholders.

            (c)    WITHDRAWAL IN REGISTRATION. If the Purchaser or a holder of
other securities entitled (upon request ) to be included in such Registration,
disapproves of the terms of any such underwriting, the Purchaser or such holder
may elect to withdraw therefrom by written notice to the Company and the
Underwriter's Representative delivered at least seven (7) days prior to the
effective date of the Registration Statement. Any Warner Registrable Shares or
other securities excluded or withdrawn from such underwriting shall be withdrawn
from such Registration.

       6.5  BLUE SKY IN REGISTRATION. In the event of any Registration of Warner
Registrable Shares pursuant to Article 6, the Company will exercise its best
efforts to Register and qualify the securities covered by the Registration
Statement under such other securities or Blue Sky laws of such jurisdictions as
the Purchaser shall reasonably request and as shall be reasonably appropriate
for the distribution of such securities; provided, however, that the Company
shall not be required to qualify to do business or to file a general consent to
service of process in any such states or jurisdictions.

       6.6  TERMINATION OF REGISTRATION RIGHTS. All registration rights granted
under this Article 6 shall terminate and be of no further force and effect five
(5) years after the Effective Date. In addition, the Purchaser's registration
rights under this Article 6 shall expire if all Warner Registrable Securities
held by and issuable to the Purchaser may be sold under Rule 144 during any
ninety (90) day period.

   7.  MISCELLANEOUS.

       7.1   GOVERNING LAW. This Agreement shall be governed by and interpreted
under the laws of the State of New York.

       7.2  SUCCESSORS AND ASSIGNS. Except as otherwise expressly provided
herein, the provisions hereof shall inure to the benefit of, and be binding
upon, the successors, assigns, heirs, executors, and administrators of the
parties hereto.

       7.3  ENTIRE AGREEMENT. This Agreement, the Collaboration Agreement and
the other documents delivered pursuant hereto, constitutes the full and entire
understanding and agreement among the parties with regard to the subjects hereof
and no party shall be liable or bound to any other party in any manner by any
representations, warranties, covenants, or agreements except as specifically set
forth herein or therein. Nothing in this Agreement, express or implied, is
intended to confer upon any party, other than the parties hereto and their
respective successors and assigns, any rights, remedies, obligations, or
liabilities under or by reason of this Agreement, except as expressly provided
herein.

                                 9

<PAGE>

       7.4  SEVERABILITY. In the event any provision of this Agreement shall be
invalid, illegal, or unenforceable, it shall to the extent practicable, be
modified so as to make it valid, legal and enforceable and to retain as nearly
as practicable the intent of the parties, and the validity, legality, and
enforceability of the remaining provisions shall not in any way be affected or
impaired thereby.

       7.5  AMENDMENT AND WAIVER. Except as otherwise provided herein, any term
of this Agreement may be amended and the observance of any term of this
Agreement may be waived (either generally or in a particular instance, either
retroactively or prospectively, and either for a specified period of time or
indefinitely), with the written consent of the Company and the Purchaser. Any
amendment or waiver effected in accordance with this Section shall be binding
upon any holder of any securities purchased under this Agreement (including
securities into which such securities have been converted), each future holder
of all such securities, and the Company.

       7.6  NOTICES. Subject to the final sentence of this Section 7.6, all
notices and other communications required or permitted hereunder shall be in
writing and shall be deemed effectively given and received (a) upon personal
delivery, (b) on the fifth day following mailing by registered or certified
mail, return receipt requested, postage prepaid, addressed to the Company and
the Purchaser at their respective addresses first above written, (c) upon
transmission of telegram or facsimile (with telephonic notice), or (d) upon
confirmed delivery by overnight commercial courier service. Any Put Notification
shall be effective only upon receipt by Purchaser (which may be by facsimile to
a facsimile number specified in writing by the Purchaser, with telephonic
notice).

       7.7  FEES AND EXPENSES. The Company and the Purchaser shall bear their
own expenses and legal fees incurred on their behalf with respect to this
Agreement and the transactions contemplated hereby.

       7.8  TITLES AND SUBTITLES. The titles of the sections and subsections of
this Agreement are for convenience of reference only and are not to be
considered in construing this Agreement.

       7.9  COUNTERPARTS. This Agreement may be executed in any number of
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one instrument.

                                   10
<PAGE>

         The foregoing Agreement is hereby executed as of the date first above
written.

"COMPANY"                              ONYX PHARMACEUTICALS, INC.

                                       By: /s/ Hollings C. Renton
                                          -------------------------------------
                                       Name:   Hollings C. Renton
                                            -----------------------------------
                                       Title:  President & CEO
                                             ----------------------------------

"PURCHASER"                            WARNER-LAMBERT COMPANY

                                       By: /s/ Anthony Wild, Ph.D.
                                          -------------------------------------
                                       Name:   Anthony Wild, Ph.D.
                                            -----------------------------------
                                       Title:  President, Parke-Davis
                                             ----------------------------------

                                     11<PAGE>
                                                                   EXHIBIT 10.28

                           ONYX PHARMACEUTICALS, INC.

                            STOCK PURCHASE AGREEMENT

                                JANUARY 18, 2000

<PAGE>

                                TABLE OF CONTENTS

<TABLE>
<CAPTION>
                                                                                   PAGE
<S>               <C>                                                              <C>
SECTION 1.        AUTHORIZATION OF SALE OF THE SECURITIES............................1

SECTION 2.        AGREEMENT TO SELL AND PURCHASE THE SHARES..........................1

         2.1      Sale of Shares.....................................................1

SECTION 3.        CLOSING AND DELIVERY...............................................1

         3.1      Closing............................................................1

         3.2      Delivery of the Shares.............................................1

SECTION 4.        REPRESENTATIONS, WARRANTIES AND COVENANTS OF THE COMPANY...........2

         4.1      Organization and Qualification.....................................2

         4.2      Due Execution, Delivery and Performance of the Documents...........2

         4.3      No Conflicts.......................................................2

         4.4      Governmental Consents..............................................3

         4.5      Issuance and Sale of the Shares....................................3

         4.6      SEC Reports........................................................3

         4.7      No Material Change.................................................4

         4.8      Capitalization.....................................................4

         4.9      Nasdaq Market......................................................4

         4.10     Absence of Litigation..............................................4

         4.11     Intangible Rights..................................................4

         4.12     Legal Compliance...................................................5

         4.13     Certain Agreements.................................................5

         4.14     Securities Act Exemption...........................................5

         4.15     Brokers............................................................5

         4.16     No Preemptive Rights...............................................5

SECTION 5.        REPRESENTATIONS, WARRANTIES AND COVENANTS OF THE PURCHASER.........6

         5.1      Authority, Approval and Enforceability.............................6

         5.2      Investment Representations.........................................6

SECTION 6.        SURVIVAL OF REPRESENTATIONS, WARRANTIES AND AGREEMENTS.............7

SECTION 7.        CONDITIONS TO COMPANY'S OBLIGATIONS AT THE CLOSING.................7

         7.1      Representations and Warranties Correct.............................8

         7.2      Covenants Performed................................................8
</TABLE>

                                       i

<PAGE>

                                TABLE OF CONTENTS
                                   (CONTINUED)

<TABLE>
<CAPTION>
                                                                                   PAGE
<S>               <C>                                                              <C>
         7.3      Qualifications.....................................................8

         7.4      Legal Investment...................................................8

SECTION 8.        CONDITIONS TO PURCHASERS' OBLIGATIONS AT THE CLOSING...............8

         8.1      Representations and Warranties Correct.............................8

         8.2      Legal Opinion......................................................8

         8.3      Covenants Performed................................................8

         8.4      Qualifications.....................................................8

         8.5      Legal Investment...................................................9

         8.6      Compliance Certificate.............................................9

SECTION 9.        REGISTRATION OF THE SHARES; COMPLIANCE WITH THE SECURITIES ACT.....9

         9.1      Definitions........................................................9

         9.2      Registration Procedures and Expenses...............................9

         9.3      Piggyback Registrations...........................................11

         9.4      Indemnification...................................................12

         9.5      Transfer of Shares After Registration; Notice.....................13

         9.6      Reporting Requirements............................................14

         9.8      Termination of Obligations........................................14

         9.9      Assignability of Registration Rights..............................14

SECTION 10.       BROKER'S FEE......................................................14

SECTION 11.       NOTICES...........................................................15

SECTION 12.       MISCELLANEOUS.....................................................15

         12.1     Waivers and Amendments............................................15

         12.2     Headings..........................................................15

         12.3     Severability......................................................15

         12.4     Governing Law.....................................................15

         12.5     Counterparts......................................................15

         12.6     Successors and Assigns............................................16

         12.7     Expenses..........................................................16

         12.8     Entire Agreement..................................................16

         12.9     Publicity.........................................................16

12.10    WAIVER OF CONFLICTS........................................................16
</TABLE>

                                       ii

<PAGE>

                           ONYX PHARMACEUTICALS, INC.

                            STOCK PURCHASE AGREEMENT

         THIS AGREEMENT ("Agreement") is made as of the 18th day of January,
2000 (the "Effective Date"), by and among ONYX PHARMACEUTICALS, INC., a
Delaware corporation (the "Company"), and each of those persons and entities,
severally and not jointly, set forth on the Schedule of Purchasers attached
as EXHIBIT A hereto (which persons and entities are hereinafter collectively
referred to herein as "Purchasers" and each individually as a "Purchaser").

                                    AGREEMENT

         In consideration of the mutual covenants contained in this Agreement
and for other good and valuable consideration, the receipt of which is hereby
acknowledged, the Company and each Purchaser (severally and not jointly)
hereby agree as follows:

SECTION 1. AUTHORIZATION OF SALE OF THE SECURITIES. Subject to the terms and
conditions of this Agreement, the Company has or before the Closing (as
defined below) will have authorized the sale and issuance of up to 2,000,000
shares of the Company's Common Stock, $0.001 par value (the "Shares").

SECTION 2. AGREEMENT TO SELL AND PURCHASE THE SHARES.

     2.1 SALE OF SHARES. At the Closing (as defined in Section 3), the
Company shall issue and sell to each Purchaser, severally and not jointly,
and each Purchaser shall purchase from the Company, severally and not
jointly, the number of Shares set forth next to such Purchaser's name on the
Schedule of Purchasers attached hereto as EXHIBIT A (the "Schedule of
Purchasers") at a purchase price of $ 9.00 per share (subject to
proportionate adjustment upon the occurrence of any stock split, stock
dividend, reverse stock split or like event that is consummated or becomes
effective during the period commencing on the date hereof and ending
immediately prior to the Closing).

SECTION 3. CLOSING AND DELIVERY

     3.1 CLOSING. The closing of the purchase and sale of the Shares to be
issued pursuant to this Agreement (the "Closing") shall be held at the
offices of Cooley Godward LLP, Five Palo Alto Square, 3000 El Camino Real,
Palo Alto, California, on January 18, 2000 or on such other date and place as
may be agreed to by the Company and the Purchasers.

     3.2 DELIVERY OF THE SHARES. Promptly following the Closing, but in no
event later than five days following the Closing, the Company shall deliver
to each Purchaser certificates representing the number of shares to be
purchased at the Closing by each Purchaser registered in the name of such
Purchaser, or in such nominee name(s) as designated by such Purchaser against
payment of the purchase price therefore by wire transfer. The Company shall
also deliver an executed copy of this Agreement to each Purchaser.

                                       1.

<PAGE>

SECTION 4. REPRESENTATIONS, WARRANTIES AND COVENANTS OF THE COMPANY.

         Subject to and except as set forth on the Schedule of Exceptions
which is arranged in Sections corresponding to the sub-section numbered
provisions contained below in this Section and except as described in the SEC
Reports (as defined below), the Company hereby represents and warrants to,
and covenants with, the Purchasers as of the Closing as follows:

     4.1 ORGANIZATION AND QUALIFICATION. The Company is a corporation duly
organized, validly existing and in good standing under the laws of the State
of Delaware and has all requisite corporate power and authority and all
licenses, permits and authorizations to conduct its business as it is
currently being conducted and as it is presently proposed to be conducted and
to own, lease and operate its properties. True and complete copies of the
Restated Certificate of Incorporation and the Bylaws of the Company, as
amended to and as in effect on the date hereof, have been delivered to the
Investors as certified by the Company's Secretary. The Company is duly
qualified and is authorized to transact business and is in good standing as a
foreign corporation in each jurisdiction in which the failure so to qualify
would have a Material Adverse Effect. As used in this Agreement, a "Material
Adverse Effect" means (a) a material adverse effect upon the business,
operations, properties, assets or condition (financial or otherwise) of the
Company or, as the case may be, the Company and any of its subsidiaries,
taken as a whole or (b) the impairment of the ability of the Company to
perform its obligations under this Agreement or any of the Documents (defined
below in Section 4.2).

     4.2 DUE EXECUTION, DELIVERY AND PERFORMANCE OF THE DOCUMENTS. The
Company's execution, delivery and performance of this Agreement and the
management rights letter attached hereto as Exhibit B (the "Management Rights
Letter") and the issuance and sale of the Shares have been duly authorized by
all requisite corporate and stockholder action by the Company and its
stockholders, respectively. Upon the execution and delivery by the Company,
and assuming the valid execution and delivery of this Agreement and the
Management Rights Letter (collectively, the "Documents") by each of the
Purchasers, the Documents will constitute valid and binding obligations of
the Company, enforceable in accordance with each of their respective terms,
except as enforceability may be limited by applicable bankruptcy, insolvency,
reorganization, moratorium or similar laws affecting creditors' and
contracting parties' rights generally and except as enforceability may be
subject to general principles of equity (regardless of whether such
enforceability is considered in a proceeding in equity or at law), including
specific performance, and except as the indemnification provisions contained
in Section 9.4 hereof may be legally unenforceable.

     4.3 NO CONFLICTS. The Company's execution, delivery and performance of
the Documents will not violate, conflict with, result in a breach of or
constitute (upon notice or lapse of time or both) a default under, or result
in the creation or imposition of any lien, security interest, mortgage,
pledge, charge or other encumbrance, of any material nature, upon any
properties or assets of the Company under any (a) law, regulation, rule,
injunction, judgment, order, decree, ruling, charge or other restriction of
any government, governmental agency, court or arbitrator to which the Company
is subject, (b) the Company's Amended and Restated Certificate of
Incorporation or Bylaws of the Company or (c) any provision of any material
indenture, mortgage, agreement, contract or other material instrument to
which the Company is a party or by which the Company or any of its properties
or assets is bound as of the date hereof.

                                       2.

<PAGE>

     4.4 GOVERNMENTAL CONSENTS. Except for applicable filings with The Nasdaq
Stock Market, Inc. (the "Nasdaq Market"), under the Securities Act of 1933,
as amended (the "Securities Act"), or the Securities Exchange Act of 1934, as
amended (the "Exchange Act"), no consent, approval, qualification, order or
authorization of, or filing with, any local, state, or federal governmental
authority is required on the part of the Company in connection with the
Company's valid execution, delivery, or performance of the Documents, or the
offer, sale or issuance of the Shares by the Company, other than any
post-closing filings as may be required under applicable federal or state
securities laws, which will be timely filed within the applicable periods
therefor.

     4.5 ISSUANCE AND SALE OF THE SHARES. When issued and paid for in
accordance with this Agreement, the Shares to be sold hereunder by the
Company will be validly issued and outstanding, fully paid and non-assessable.

     4.6  SEC REPORTS.

          (a) Since January 1, 1999, the Company has filed in a timely manner
with the Securities and Exchange Commission (the "SEC") all reports ("SEC
Reports") required to be filed by it under the Securities Exchange Act of
1934, as amended (the "Exchange Act"). All of the SEC Reports filed by the
Company comply in all material respects with the requirements of the Exchange
Act. None of the SEC Reports contains, as of the respective dates thereof,
any untrue statement of a material fact or omits to state any material fact
required to be stated therein or necessary to make the statements therein not
misleading in light of the circumstances under which they were made. All
financial statements contained in the SEC Reports have been prepared in
accordance with generally accepted accounting principles consistently applied
throughout the period indicated ("GAAP"). Each balance sheet is in accordance
with the books and records of the Company and presents fairly in accordance
with GAAP the financial position of the Company as of the date of such
balance sheet, and each statement of operations, of stockholders' equity and
of cash flows is in accordance with the books and records of the Company and
presents fairly in accordance with GAAP the results of operations, the
stockholders' equity and the cash flows of the Company for the periods then
ended.

          (b) The Company has delivered to the Purchasers the following SEC
Reports:

               (i)  the Company's  Annual Report on Form 10-K for the fiscal
year ended December 31, 1998 (without exhibits);

               (ii) the Company's Quarterly Reports on Form 10-Q as filed
with the SEC for the quarters ended March 31, 1999, June 30, 1999 and
September 30, 1999 (without exhibits);

               (iii) the Company's Proxy Statement for the 1999 Annual
Meeting of Stockholders.

          (c) No event has occurred since January 1, 1999, requiring the
filing of an SEC Report that has not heretofore been filed and furnished to
the Purchasers (including, without limitation, any amendment to any such SEC
Report).

                                       3.

<PAGE>

     4.7 NO MATERIAL CHANGE. As of the date hereof, there has been no event
or action that would have a Material Adverse Effect since September 30, 1999,
except that the Company continues to incur losses as described in the SEC
Reports.

     4.8 CAPITALIZATION. The authorized capital stock of the Company consists
of (i) 25,000,000 shares of Common Stock, $.001 par value, of which
11,558,110 such shares were issued and outstanding as of December 31, 1999
and (ii) 5,000,000 shares of preferred stock, $.001 par value, of which no
shares are issued and outstanding on the date hereof. As of the date hereof,
the Company has no intention, obligation or commitment, fixed or contingent,
to issue any shares of such Preferred Stock. Except as contemplated by this
Agreement and except for 2,351,146 shares reserved under the 1996 Equity
Incentive Plan, the 1996 Non-Employee Directors' Stock Option Plan and the
1996 Employee Stock Purchase Plan as of December 31, 1999, there are no
existing options, warrants, calls, preemptive (or similar) rights,
subscriptions or other rights, agreements, arrangements or commitments of any
character obligating the Company to issue, transfer or sell, or cause to be
issued, transferred or sold, any shares of capital stock of the Company or
other equity interests in the Company or any securities convertible into or
exchangeable for such shares of capital stock or other equity interests, and
there are no outstanding contractual obligations of the Company to
repurchase, redeem or otherwise acquire, or prepare and file with the SEC any
registration statement to register under the Securities Act of 1933, as
amended (the "Securities Act") with respect to, any such shares of capital
stock or other equity interests.

     4.9 NASDAQ MARKET. The Company's Common Stock is listed on the Nasdaq
Market under the trading symbol "ONXX," and there are no proceedings to
revoke or suspend such listing.

     4.10 ABSENCE OF LITIGATION. There is no action, suit, proceeding or
investigation pending or, to the Company's best knowledge, that has been
filed, commenced or threatened, by or before any governmental agency, court
or arbitrator against the Company which might result have either individually
or in the aggregate, a Material Adverse Effect (including, without
limitation, any such action, suit, proceeding or investigation that questions
the validity of this Agreement or the issuance of the Shares thereunder).

     4.11 INTANGIBLE RIGHTS. To the Company's best knowledge, the Company
owns or has the right to use pursuant to valid and enforceable licenses,
sublicenses, agreements or permissions, all Intangible Rights (as defined
below) that are necessary or desirable for the conduct of the business of the
Company as it is currently being conducted and as it is presently proposed to
be conducted, and no claims adverse to the interests of the Company are
pending or, to the best knowledge of the Company, have been threatened or
otherwise asserted with respect to the Company's ownership or use of any such
Intangible Rights. To the Company's best knowledge, the Company is not
infringing any Intangible Right owned or used by any third party nor, to the
Company's best knowledge, is any third party infringing any Intangible Right
owned or used by the Company. For purposes of this Agreement, the term
"Intangible Rights" means (i) all inventions (whether patentable or
unpatentable, and whether or not reduced to practice), all improvements
thereto, and all patents, patent applications, and patent disclosures,
together with all reissuances, continuations, continuations-in-part,
revisions, extensions, and reexaminations thereof, (ii) all trademarks,
service marks, trade dress, logos, trade names and corporate names,

                                       4.

<PAGE>

together with all translations, adaptations, derivations and combinations
thereof and including all goodwill associated therewith, and all
applications, registrations and renewals in connection therewith, (iii) all
copyrightable works, all copyrights, all applications, registrations and
renewals in connection therewith, (iv) all trade secrets and confidential
business information (including, without limitation, ideas, research and
development, know-how, formulas, compositions, manufacturing and production
processes and techniques, technical data, designs, drawings, specifications,
supplier lists, and business and marketing plans and proposals), (v) all
computer software (including, data and related documentation), (vi) all other
proprietary rights and (vii) all copies and tangible embodiments of any of
the foregoing (in whatever form or medium).

     4.12 LEGAL COMPLIANCE. The Company is not in default or violation of its
Restated Certificate of Incorporation or Bylaws and has not violated any
applicable laws (including, without limitation, all rules, regulations,
codes, plans, injunctions, judgments, orders, decrees, rulings and charges
thereunder) of federal, state, local and foreign governments (and all
agencies thereof) in respect of the conduct of its business or the ownership
of its properties which default violation would (either individually or in
the aggregate) have a Material Adverse Effect. To the knowledge of the
Company, there exists no condition, event or act which constitutes, or which
after notice, lapse of time or both, would constitute, such a default or
violation under any of the foregoing except where such a default is not
reasonably expected to have a Material Adverse Effect.

     4.13 CERTAIN AGREEMENTS. All of the collaborative agreements, research
and development agreements, licensing agreements and other agreements with
corporate partners and governmental or educational entities that have been
previously disclosed by the Company in the SEC Reports referred to in
paragraph (b) of Section 4.6 hereof are valid and enforceable obligations of
the Company and, to the Company's best knowledge, the other parties thereto.
Except for breaches and defaults that would not, singly or in the aggregate,
have a Material Adverse Effect, the Company is not in breach or default under
any such contracts or agreements nor has any event occurred which, with the
giving of notice or the passage of time or both, would constitute a breach or
default on the Company's part thereunder. To the Company's best knowledge,
none of the other parties to such contracts or agreements is in breach or
default thereunder nor has any event occurred which, with the giving of
notice or the passage of time or both, would constitute a breach or default
on such other parties' part that would have a Material Adverse Effect.

     4.14 SECURITIES ACT EXEMPTION. Assuming and relying in part on the truth
and accuracy of Purchasers' representations and warranties in Section 5 of
this Agreement, the offer, sale and issuance of the Common Stock is exempt
from registration under the Securities Act.

     4.15 BROKERS. Neither the Company nor any of the officers, directors,
employees or stockholders of the Company has employed any broker or finder in
connection with the transaction contemplated by this Agreement.

     4.16 NO PREEMPTIVE RIGHTS. There exist no statutory preemptive, or other
similar rights to purchase securities of the Company.

                                       5.

<PAGE>

SECTION 5. REPRESENTATIONS, WARRANTIES AND COVENANTS OF THE PURCHASER.

     Each Purchaser, severally and not jointly, represents and warrants
to and covenants with the Company that:

     5.1 AUTHORITY, APPROVAL AND ENFORCEABILITY.

          (a) Purchaser has full power and authority to execute, deliver and
perform its obligations under this Agreement and all agreements, instruments
and documents contemplated hereby, and all action of Purchaser necessary for
such execution, delivery and performance has been duly taken.

          (b) Purchaser's execution, delivery and performance of this
Agreement have been duly authorized by all requisite action by Purchaser,
respectively. Upon the execution and delivery by Purchaser, and assuming the
valid execution and delivery of this Agreement by each of the Purchaser and
the Company, this Agreement will constitute a valid and binding obligation of
Purchaser, enforceable in accordance with its terms, except as enforceability
may be limited by applicable bankruptcy, insolvency, reorganization,
moratorium or similar laws affecting creditors' and contracting parties'
rights generally and except as enforceability may be subject to general
principles of equity (regardless of whether such enforceability is considered
in a proceeding in equity or at law), including specific performance, and
except to the extent that the enforceability of the indemnification
provisions of Section 9.4 may be legally unenforceable.

     5.2 INVESTMENT REPRESENTATIONS. Purchaser understands that the Shares
have not been registered under the Securities Act. Purchaser also understands
that the Shares are being offered and sold pursuant to an exemption from
registration contained in the Securities Act based in part upon Purchaser's
representations contained in the Agreement. Purchaser hereby represents and
warrants as follows:

          (a) Purchaser has substantial experience in evaluating and
investing in private placement transactions of securities in companies
similar to the Company so that it is capable of evaluating the merits and
risks of its investment in the Company and has the capacity to protect its
own interests. Purchaser must bear the economic risk of this investment
indefinitely unless the Shares are registered pursuant to the Securities Act,
or an exemption from registration is available. Purchaser understands that
there is no assurance that any exemption from registration under the
Securities Act will be available and that, even if available, such exemption
may not allow Purchaser to transfer all or any portion of the Shares under
the circumstances, in the amounts or at the times Purchaser might propose.

          (b) Purchaser has been advised or is aware of the provisions of
Rule 144 promulgated under the Securities Act, which permits limited resale
of shares purchased in a private placement subject to the satisfaction of
certain conditions.

          (c) The Purchaser agrees that it will not sell, pledge, assign,
transfer, otherwise dispose of or reduce their risk with respect to
(collectively, "Transfer") any of the Shares unless the Transfer will be made
pursuant to an exemption from the registration requirements of the Securities
Act or pursuant to an effective registration statement under the Securities
Act and pursuant to an exemption from any applicable state securities laws or
an

                                       6.

<PAGE>

effective registration or other qualification under any applicable state
securities laws. The Purchaser understands that exemptions from such
registration requirements are limited. The Company is under no obligation to
register the Shares except as provided in Section 9.

          (d) The Purchaser acknowledges and agrees that the Shares are
subject to certain restrictions as to resale under the federal and state
securities laws. The Purchaser agrees and understands that stop transfer
instructions will be given to the transfer agent for the Shares and each
share certificate, and each certificate delivered on transfer of or in
substitution for any such certificate, shall have affixed a legend in
substantially the following form:

              "The shares represented by this certificate have not
              been registered under the Securities Act of 1933, as
              amended (the "Act"), and may not be offered, sold or
              otherwise transferred, assigned, pledged or
              hypothecated unless and until registered under the
              act or unless the Company has received an opinion of
              counsel satisfactory to the Company and its counsel
              that such registration is not required.

          (e) Purchaser is acquiring the Shares for Purchaser's own account
for investment only, and not with a view towards their distribution.

          (f) Purchaser represents that by reason of its, or of its
management's, business or financial experience, Purchaser has the capacity to
protect its own interests in connection with the transactions contemplated in
this Agreement. Further, Purchaser is aware of no publication of any
advertisement in connection with the transactions contemplated in the
Agreement.

          (g) Purchaser represents that it is an accredited investor within
the meaning of Regulation D under the Securities Act.

          (h) Purchaser has received the SEC Reports listed in Section 4.6(b)
and has had an opportunity to discuss the Company's business, management and
financial affairs with directors, officers and management of the Company and
has had the opportunity to review the Company's operations and facilities.
Purchaser has also had the opportunity to ask questions of and receive
answers from, the Company and its management regarding the terms and
conditions of this investment.

     SECTION 6. SURVIVAL OF REPRESENTATIONS, WARRANTIES AND AGREEMENTS.
Notwithstanding any investigation made by any party to this Agreement, all
covenants, agreements, representations and warranties made by the Company and
each Purchaser herein shall survive the execution of this Agreement and the
issuance and sale to the Purchasers of the Shares and shall terminate upon
the subsequent transfer of the Shares pursuant to Sections 5 or 9.

     SECTION 7. CONDITIONS TO COMPANY'S OBLIGATIONS AT THE CLOSING. The
Company's obligation to complete the sale and issuance of the Shares at
Closing shall be subject to the following conditions to the extent not waived
by the Company:

                                       7.

<PAGE>

     7.1  REPRESENTATIONS AND WARRANTIES CORRECT. The representations and
warranties made by each Purchaser in Section 5 hereof shall be true and
correct when made, and shall be true and correct on the Closing Date.

     7.2  COVENANTS PERFORMED. All covenants, agreements and conditions
contained in any Documents to be performed by the Purchasers on or prior to
the Closing Date shall have been performed or complied with in all material
respects.

     7.3  QUALIFICATIONS. All authorizations, approvals, or permits, if any,
of any governmental authority or regulatory body of the United States or of
any state that are binding upon any of the Purchasers and that are required
in connection with the lawful sale and issuance of the Shares at such Closing
pursuant to this Agreement shall have been duly obtained and shall be
effective on and as of the date of such Closing. No stop order or other order
enjoining the sale of the Shares shall have been issued and no proceedings
for such purpose shall be pending or, to the knowledge of the Company,
threatened by the SEC or any commissioner of corporations or similar officer
of any state having jurisdiction over this transaction.

     7.4  LEGAL INVESTMENT. At the time of such Closing, the sale and
issuance of the Shares to be purchased and sold at such Closing shall be
legally permitted by all laws and regulations to which the Purchaser and the
Company are subject.

SECTION 8. CONDITIONS TO PURCHASERS' OBLIGATIONS AT THE CLOSING. Each
Purchaser's obligation to purchase the Shares at the Closing thereby shall be
subject to the following conditions to the extent not waived by the
Purchasers:

     8.1  REPRESENTATIONS AND WARRANTIES CORRECT. The representations and
warranties made by the Company in Section 4 hereof shall be true and correct
when made, and shall be true and correct as of the Closing Date.

     8.2  LEGAL OPINION. Purchasers shall have received from Cooley Godward
LLP, counsel to the Company, an opinion letter addressed to the Purchasers,
dated as of the Closing Date, in the form attached hereto as EXHIBIT B.

     8.3  COVENANTS PERFORMED. All covenants, agreements and conditions
contained herein to be performed by the Company on or prior to the Closing
Date shall have been performed or complied with in all material respects.

     8.4  QUALIFICATIONS. All authorizations, approvals, or permits, if any,
of any governmental authority or regulatory body of the United States or of
any state that are binding upon the Company and that are required in
connection with the lawful sale and issuance of the Shares at such Closing
pursuant to this Agreement shall have been duly obtained and shall be
effective on and as of the Closing Date. No stop order or other order
enjoining the sale of the Shares shall have been issued and no proceedings
for such purpose shall be pending or, to the knowledge of the Company,
threatened by the SEC, or any commissioner of corporations or similar officer
of any state having jurisdiction over this transaction.

                                       8.

<PAGE>

     8.5  LEGAL INVESTMENT. At the time of the Closing, the sale and issuance
of the Shares shall be legally permitted by all laws and regulations to which
the Purchaser and the Company are subject.

     8.6  COMPLIANCE CERTIFICATE. The Company shall have delivered to
Purchasers, a Compliance Certificate, executed by the Chief Executive Officer
of the Company, dated the Closing Date, to the effect that the conditions,
specified in Sections 8.1, 8.3 and 8.4 have been satisfied.

SECTION 9. REGISTRATION OF THE SHARES; COMPLIANCE WITH THE SECURITIES ACT.

     9.1  DEFINITIONS. As used in this Section 9 the following terms shall
have the following respective meanings:

          (a) "Holders" shall mean the holders of Registrable Securities (as
defined therein) pursuant to the Amended and Restated Information and
Registration Rights Agreement dated May 16, 1994, between the Company and the
Investors (as defined therein), as amended (the "Registration Agreement"),
who continue to have registration rights pursuant to the Registration
Agreement.

          (b) "Registrable Shares" shall mean (i) the Shares issued pursuant
to this Agreement and (ii) any other shares of Common Stock issued in respect
to the Shares (because of stock splits, stock dividends, reclassifications,
recapitalizations, or similar events);

          (c) "Registration Statement" shall mean any registration statement
and shall include any preliminary prospectus, final prospectus, exhibit,
supplement or amendment included in or relating to the Registration Statement
referred to in Section 9.2 and Section 9.3; and

          (d) "Untrue Statement" shall include any untrue statement or
alleged untrue statement, or any omission or alleged omission to state in the
Registration Statement a material fact required to be stated therein or
necessary to make the statements therein, in the light of the circumstances
under which they were made, not misleading.

     9.2  REGISTRATION PROCEDURES AND EXPENSES.  The Company is obligated to
do the following:

     The Company shall, within 90 days immediately following the Closing
Date, such actual date being referred to as the "Registration Date":

          (a) prepare and file with the SEC a registration statement on Form
S-3 in order to register with the SEC under the Securities Act a sale by the
Purchasers on a delayed or continuous basis pursuant to Rule 415 under the
Securities Act any or all of the Registrable Shares through the automated
quotation system of the Nasdaq National Market System or the facilities of
any national securities exchange on which the Company's Common Stock is then
traded, or in privately-negotiated transactions (a "Registration Statement")
(notwithstanding anything to the contrary expressed or implied herein, if a
registration statement on Form S-3, or any substitute form, is not then
available for registration of the Registrable Shares, the Company

                                       9.

<PAGE>

shall be obligated instead to prepare and file with the SEC a registration
statement on Form S-1 in order to register the Registrable Shares under the
Securities Act and such registration statement will be a "Registration
Statement" for the purposes of this Agreement);

          (b) use its best efforts, subject to receipt of necessary
information from the Purchasers, to cause such Registration Statement to
become effective as promptly after the Registration Date as practicable, but
no later than 60 days after the Registration Date (the "Effective Date") and
take all other reasonable actions necessary under any federal law or
regulation to permit all Registrable Shares to be sold or otherwise disposed
of;

          (c) promptly notify each Purchaser, at any time when a prospectus
relating to such Registration Statement is required to be delivered under the
Securities Act, of the happening of any event as a result of which the
prospectus included in or relating to such Registration Statement contains an
untrue statement of a material fact or omits to state any fact necessary to
make the statements therein not misleading;

          (d) promptly prepare and file with the SEC, and deliver to each
Purchaser, such amendments and supplements to such Registration Statement and
the prospectus used in connection therewith as may be necessary to keep such
Registration Statement effective until termination of such obligation as
provided in Section 9.9 below;

          (e) furnish to each Purchaser such number of copies of prospectuses
in conformity with the requirements of the Securities Act, in order to
facilitate the public sale or other disposition of all or any of the
Registrable Shares by the Purchasers;

          (f) no later than the Registration Date, file such documents as may
be required of the Company for normal state securities law clearance for the
resale of the Registrable Shares in which states of the United States as may
be reasonably requested by each Purchaser provided, however, that the Company
shall not be required in connection with this paragraph (f) to qualify as a
foreign corporation or execute a general consent to service of process in any
jurisdiction;

          (g) no later than the Registration Date, use its best efforts to
cause all Registrable Shares to be listed on each securities exchange, if
any, on which equity securities by the Company are then listed;

          (h) bear all expenses in connection with the procedures in Section
9.2 and in Section 9.3, other than (i) fees and expenses, if any, of counsel
or other advisers to the Purchasers, and (ii) any expenses relating to the
sale of the Registrable Shares by the Purchasers, including broker's
commission, discounts or fees and transfer taxes; and

          (i) in addition to any other remedies at law or in equity and not
in limitation thereof, the Company shall pay the Purchasers an aggregate of
$100,000 for each 30 days (or fraction thereof) after the Effective Date that
such Registration Statement together with any applicable Nasdaq Market and
state securities law filings are not effective to permit the lawful public
resale of all the Registrable Shares.

                                       10.

<PAGE>

     9.3 PIGGYBACK REGISTRATIONS. The Company shall notify the Purchasers in
writing at least twenty (20) days prior to the filing of any registration
statement under the Securities Act for purposes of a public offering of
securities of the Company (including, but not limited to, registration
statements relating to secondary offerings of securities of the Company, but
excluding registration statements relating to employee benefit plans or with
respect to corporate reorganizations or other transactions under Rule 145 of
the Securities Act) and will afford each Purchaser an opportunity to include
in such registration statement all or part of such Registrable Shares held by
such Purchaser, including the Registrable shares previously registered
pursuant to Section 9.2 which have not been sold by the Purchaser. Each
Purchaser desiring to include in any such registration statement all or any
part of the Registrable Shares held by it shall, within twenty (20) days
after the above-described notice from the Company, so notify the Company in
writing. Such notice shall state the intended method of disposition of the
Registrable Shares by such Purchaser. If a Purchaser decides not to include
all of its Registrable Shares in any registration statement thereafter filed
by the Company, such Purchaser shall nevertheless continue to have the right
to include any Registrable Shares in any subsequent registration statement or
registration statements as may be filed by the Company with respect to
offerings of its securities, all upon the terms and conditions set forth
herein.

          (a) UNDERWRITING. If the registration statement under which the
Company gives notice under this Section 9.3 is for an underwritten offering,
the Company shall so advise the Purchasers. In such event, the right of any
such Purchaser to be included in a registration pursuant to this Section 9.3
shall be conditioned upon such Purchaser participation in such underwriting
and the inclusion of such Purchaser Registrable Shares in the underwriting to
the extent provided herein. All Purchasers proposing to distribute their
Registrable Shares through such underwriting shall enter into an underwriting
agreement in customary form with the underwriter or underwriters selected for
such underwriting by the Company. Notwithstanding any other provision of the
Agreement, if the underwriter determines in good faith that marketing factors
require a limitation of the number of shares to be underwritten, the number
of shares that may be included in the underwriting shall be allocated, first,
to the Company; second, to the Holders on a PRO RATA basis based on the total
number of Registrable Securities (as defined in the Registration Agreement)
held by the Holders; third, to the Purchasers on a PRO RATA basis based on
the total number of Registrable Shares held by the Purchasers; and fourth,
any shareholder of the Company (other than a Holder or a Purchaser) on a PRO
RATA basis. If any Purchaser disapproves of the terms of any such
underwriting, such Purchaser may elect to withdraw therefrom by written
notice to the Company and the underwriter, delivered at least ten (10)
business days prior to the effective date of the registration statement. Any
Registrable Shares excluded or withdrawn from such underwriting shall be
excluded and withdrawn from the registration. For any Purchaser which is a
partnership, corporation or limited liability company, the partners, retired
partners, shareholders, members or retired members of such Purchaser, or the
estates and family members of any such partners and retired partners and any
trusts for the benefit of any of the foregoing person shall be deemed to be a
single "Purchaser", and any PRO RATA reduction with respect to such
"Purchaser" shall be based upon the aggregate amount of shares carrying
registration rights owned by all entities and individuals included in such
"Purchaser," as defined in this sentence.

          (b) RIGHT TO TERMINATE REGISTRATION. The Company shall have the
right to terminate or withdraw any registration initiated by it under this
Section 9.3 prior to the

                                       11.

<PAGE>

effectiveness of such registration whether or not any Purchaser has elected
to include securities in such registration. The Registration Expenses of such
withdrawn registration shall be borne by the Company in accordance with
Section 9.2(h) hereof.

     9.4  INDEMNIFICATION

          (a) The Company agrees to indemnify and hold harmless each
Purchaser and underwriter (and each person, if any, who controls the
Purchaser within the meaning of Section 15 of the Securities Act) from and
against any losses, claims, damages or liabilities to which such Purchaser
(or such underwriter or controlling person) may become subject (under the
Securities Act or otherwise) insofar as such losses, claims, damages or
liabilities (or actions or proceedings in respect thereof) arise out of, or
are based upon, any Untrue Statement contained in the Registration Statement
on the Effective Date thereof, or arise out of any failure by the Company to
fulfill any undertaking included in the Registration Statement and the
Company will reimburse such Purchaser (or such underwriter or controlling
person) for any reasonable legal or other expenses reasonably incurred in
investigating, defending or preparing to defend any such action, proceeding
or claim; PROVIDED, HOWEVER, that the Company shall not be liable in any such
case to the extent that such loss, claim, damage or liability arises out of,
or is based upon, an Untrue Statement made in such Registration Statement in
reliance upon and in conformity with written information furnished to the
Company by or on behalf of such Purchaser specifically for use in preparation
of the Registration Statement, or the failure of such Purchaser to comply
with the covenants and agreements contained in Section 9.5 hereof respecting
the sale of the Registrable Shares or any statement or omission in any
prospectus that is corrected in any subsequent prospectus that was delivered
to the Purchaser prior to the pertinent sale or sales by the Purchaser.

          (b) Each Purchaser, severally and not jointly, agrees to indemnify
and hold harmless the Company and underwriter (and each person, if any, who
controls the Company within the meaning of Section 15 of the Securities Act,
each officer of the Company who signs the Registration Statement and each
director of the Company) from and against any losses, claims, damages or
liabilities to which the Company (or any such underwriter, officer, director
or controlling person) may become subject (under the Securities Act or
otherwise), insofar as such losses, claims, damages or liabilities (or
actions or proceedings in respect thereof) arise out of, or are based upon,
any failure to comply with the covenants and agreements contained in Section
9.5 hereof respecting sale of the Registrable Shares, or any Untrue Statement
contained in the Registration Statement on the Effective Date thereof if such
Untrue Statement was made in reliance upon and in conformity with written
information furnished by or on behalf of such Purchaser specifically for use
in preparation of the Registration Statement, and such Purchaser will
reimburse the Company (or such underwriter, officer, director or controlling
person), as the case may be, for any legal or other expenses reasonably
incurred in investigating, defending or preparing to defend any such action,
proceeding or claim; PROVIDED that in no event shall any indemnity by a
Purchaser under this Section 9.4 exceed the net proceeds received by such
Purchaser from the sale of the Registrable Shares covered by such
Registration Statement.

          (c) Promptly after receipt by any indemnified person of a written
notice of a claim or the beginning of any action in respect of which
indemnity is to be sought against an indemnifying person pursuant to this
Section 9.4, such indemnified person shall notify the

                                       12.

<PAGE>

indemnifying person in writing of such claim or of the commencement of such
action, and, subject to the provisions hereinafter stated, in case any such
action shall be brought against an indemnified person and such indemnifying
person shall have been notified thereof, such indemnifying person shall be
entitled to participate therein, and, to the extent it shall wish, to assume
the defense thereof, with counsel reasonably satisfactory to such indemnified
person. After notice from the indemnifying person to such indemnified person
of its election to assume the defense thereof, such indemnifying person shall
not be liable to such indemnified person for any legal expenses subsequently
incurred by such indemnified person in connection with the defense thereof;
PROVIDED, HOWEVER, that if there exists or shall exist a conflict of interest
that would make it inappropriate, in the opinion of counsel to the
indemnified person, for the same counsel to represent both the indemnified
person and such indemnifying person or any affiliate or associate thereof,
the indemnified person shall be entitled to retain its own counsel at the
expense of such indemnifying person; PROVIDED, HOWEVER, that no indemnifying
person shall be responsible for the fees and expenses of more than one
separate counsel for all indemnified parties; PROVIDED, HOWEVER, that no
person guilty of fraudulent misrepresentation (within the meaning of Section
11(f) of the Securities Act) shall be entitled to contribution from any
person who was not guilty of such fraudulent misrepresentation.

          (d) If the indemnification provided for in this Section 9.4 is held
by a court of competent jurisdiction to be unavailable to an indemnified
party with respect to any loss, liability, claim, damage, or expense referred
to therein, then the indemnifying party, in lieu of indemnifying such
indemnified party hereunder, shall contribute to the amount paid or payable
by such indemnified party as a result of such loss, liability, claim, damage,
or expense in such proportion as is appropriate to reflect the relative fault
of the indemnifying party on the one hand and of the indemnified party on the
other in connection with the statements or omissions that resulted in such
loss, liability, claim, damage, or expense as well as any other relevant
equitable considerations. The relative fault of the indemnifying party and of
the indemnified party shall be determined by reference to, among other
things, whether the untrue or alleged untrue statement of material fact or
the omission to state a material fact relates to information supplied by the
indemnifying party or by the indemnified party and the parties' relative
intent, knowledge, access to information, and opportunity to correct or
prevent such statement or omission; PROVIDED, that in no event shall any
contribution by a Purchaser hereunder exceed the net proceeds received by
such Purchaser from the sale of the Shares covered by the Registration
Statement.

          (e) Notwithstanding the foregoing, to the extent that the
provisions on indemnification and contribution in the underwriting agreement
entered into in connection with the underwritten public offering are in
conflict with the foregoing provisions, the provisions in the underwriting
agreement shall control.

     9.5 TRANSFER OF SHARES AFTER REGISTRATION; NOTICE. The Purchaser hereby
covenants with the Company not to make any sale of the Registrable Shares
after registration without effectively causing the prospectus delivery
requirement under the Securities Act to be satisfied. The Purchaser
acknowledges that there may be times when the Company must suspend the use of
the prospectus forming a part of the Registration Statement until such time
as an amendment to the Registration Statement has been filed by the Company
and declared effective by the SEC, or until such time as the Company has
filed an appropriate report with the SEC pursuant to the

                                       13.

<PAGE>

Exchange Act. The Purchaser hereby covenants that it will not sell any Shares
pursuant to said prospectus during the period commencing at the time at which
the Company gives the Purchaser written notice of the suspension of the use
of said prospectus and ending at the time the Company gives the Purchaser
notice that the Purchaser may thereafter effect sales pursuant to said
prospectus. The foregoing provisions of this Section 9.5 shall in no manner
diminish or otherwise impair the Company's obligations under Section 9.2 and
Section 9.3 hereof.

     9.6  REPORTING REQUIREMENTS.

          (a)  The Company agrees to use its best efforts to:

               (i)   make and keep public information available, as those terms
are understood and defined in Rule 144 under the Securities Act;

               (ii)  file with the SEC in a timely manner all reports and
other documents required of the Company under the Securities Act and the
Securities Exchange Act of 1934; and

               (iii) so long as any of the Purchasers own Registrable Shares,
to furnish to the Purchasers forthwith upon request (1) a written statement
by the Company as to whether it complies with the reporting requirements of
said Rule 144, the Securities Act and Securities Exchange Act of 1934, or
whether it qualifies as a registrant whose securities may be resold pursuant
to SEC Form S-3, (2) a copy of the most recent annual or quarterly report of
the Company and such other reports and documents so filed by the Company, and
(3) such other information as may be reasonably requested in availing the
Purchasers of any rule or regulation of the SEC that would permit the selling
of the Registrable Shares without registration.

     9.7  INFORMATION RIGHTS. So long as each Purchaser continues to own at
least ten (10%) of its Registrable Shares, the Company shall provide each
such Purchaser a copy of all SEC Reports promptly after filing such SEC
Report with the SEC.

     9.8  TERMINATION OF OBLIGATIONS. The obligations of the Company pursuant
to Sections 9.2 through 9.6 hereof shall cease and terminate upon the earlier
to occur of (i) such time as all of the Registrable Shares have been resold
or (ii) such time as all of the Registrable Shares may be sold during any 90
day period pursuant to Rule 144, including Rule 144 (k), without being
restricted by the volume limitations of Rule 144(e).

     9.9  ASSIGNABILITY OF REGISTRATION RIGHTS. The registration rights set
forth in this Section 9 are not assignable other than to an affiliate of a
Purchaser or, if the Purchaser is a partnership or limited liability company,
limited partner or a member of a Purchaser; PROVIDED, HOWEVER, that the
Purchaser shall only have the right to require the Company to amend the
Registration Statement twice for such assignments.

SECTION 10. BROKER'S FEE. The Company and each Purchaser (severally and not
jointly) shall indemnify each other for any broker's, finder's or agent's
fees for which they are responsible.

                                       14.

<PAGE>

SECTION 11. NOTICES. All notices, requests, consents and other communications
hereunder shall be in writing, shall be sent by confirmed facsimile or mailed
by first-class registered or certified airmail, or nationally recognized
overnight express courier, postage prepaid, and shall be deemed given when so
sent and addressed as follows:

     (a)  if to the Company, to:

                                    ONYX Pharmaceuticals, Inc.
                                    3031 Research Drive
                                    Richmond, California 94806
                                    Attention: Hollings C. Renton

          with a copy mailed to:

                                    Cooley Godward LLP
                                    Five Palo Alto Square
                                    3000 El Camino Real
                                    Palo Alto, California 94306
                                    Attention:  Robert L. Jones, Esq.

or to such other person at such other place as the Company shall designate to
the Purchasers in writing; and

          (b) if to the Purchasers, at the address as set forth at the end of
this Agreement, or at such other address or addresses as may have been
furnished to the Company in writing.

SECTION 12. MISCELLANEOUS.

     12.1 WAIVERS AND AMENDMENTS. Neither this Agreement nor any provision
hereof may be changed, waived, discharged, terminated, modified or amended
except upon the written consent of the Company and holders of at least 66 2/3%
of the Shares.

     12.2 HEADINGS. The headings of the various sections of this Agreement
have been inserted for convenience of reference only and shall not be deemed
to be part of this Agreement.

     12.3 SEVERABILITY. In case any provision contained in this Agreement
should be invalid, illegal or unenforceable in any respect, the validity,
legality and enforceability of the remaining provisions contained herein
shall not in any way be affected or impaired thereby.

     12.4 GOVERNING LAW. This Agreement shall be governed by and construed in
accordance with the laws of the State of Delaware as applied to contracts
entered into and performed entirely in Delaware by Delaware residents,
without regard to conflicts of law principles.

     12.5 COUNTERPARTS. This Agreement may be executed in two or more
counterparts, each of which shall constitute an original, but all of which,
when taken together, shall constitute but one instrument, and shall become
effective when one or more counterparts have been signed by each party hereto
and delivered to the other parties.

                                       15.

<PAGE>

     12.6 SUCCESSORS AND ASSIGNS. Except as otherwise expressly provided
herein, the provisions hereof shall inure to the benefit of, and be binding
upon, the successors, assigns, heirs, executors and administrators of the
parties hereto. The Company shall not, directly or indirectly, enter into any
merger, consolidation or reorganization in which the Company shall not be the
surviving corporation unless the proposed surviving corporation shall, prior
to such merger, consolidation or reorganization, agree in writing to assume
the obligations of the Company under this Agreement; PROVIDED, HOWEVER, that
the provisions of this Section 12.6 shall not apply in the event of any
merger, consolidation or reorganization in which the Company is not the
surviving corporation if all Purchasers are entitled to receive in exchange
for their Registrable Shares consideration consisting solely of (i) cash, or
(ii) securities of the acquiring corporation which may be immediately sold to
the public without registration under the Securities Act.

     12.7 EXPENSES. Each party shall pay all costs and expenses that it
incurs with respect to the negotiation, execution, delivery and performance
of this Agreement. The Company shall, at the Closing, reimburse the
reasonable fees of and expenses of one special counsel for the Purchasers,
and shall reimburse such special counsel for reasonable expenses incurred in
connection with the negotiation, execution, delivery and performance of this
Agreement.

     12.8 ENTIRE AGREEMENT. This Agreement, the Non-Disclosure Agreements
dated August 23, 1999 and October 8, 1999, the Management Rights Letter and
other documents delivered pursuant hereto, including the exhibits, constitute
the full and entire understanding and agreement between the parties with
regard to the subjects hereof and thereof.

     12.9 PUBLICITY. No party shall issue any press releases or otherwise
make any public statement with respect to the transactions contemplated by
this Agreement without the prior written consent of the other parties, except
as may be required by applicable law or regulations, in which case such party
shall provide the other parties with reasonable notice of such publicity
and/or opportunity to review such disclosure.

     12.10. WAIVER OF CONFLICTS. Each party to this Agreement acknowledges
that legal counsel for the Company, Cooley Godward LLP ("Cooley Godward"),
has in the past and may continue in the future to perform legal services for
one or more of the Purchasers or their affiliates in matters unrelated to the
transactions contemplated by this Agreement, including, but not limited to,
the representation of the Purchasers in matters of a similar nature to the
transactions contemplated herein. Each party to this Agreement hereby (a)
acknowledges that they have had an opportunity to ask for and have obtained
information relevant to such representation, including disclosure of the
reasonably foreseeable adverse consequences of such representation; (b)
acknowledges that with respect to the transactions contemplated herein,
Cooley Godward has represented the Company and not any individual Purchaser
or any individual shareholder, director or employee of the Company; and (c)
gives its informed consent to Cooley Godward's representation of the Company
in the transactions contemplated by this Agreement and Cooley Godward's
representation of one or more of the Purchasers or their affiliates in
matters unrelated to such transactions.

                                       16.

<PAGE>

     IN WITNESS WHEREOF, the parties hereto have caused this Agreement to
be executed by their duly authorized representatives as of the day and year
first above written.

COMPANY:                              PURCHASERS:

ONYX PHARMACEUTICALS, INC.            ALTA BIOPHARMA PARTNERS, L.P.
                                      By: Alta BioPharma Management, LLC

By: /s/ Hollings C. Renton            By: /s/ Eileen McCarthy
   ---------------------------           -------------------------------
Name:   Hollings C. Renton            Name:   Eileen McCarthy
     -------------------------             -----------------------------
Title:  President & CEO               Title: Member
      ------------------------              ----------------------------

Address: 3031 Research Drive          Address: One Embarcadero Center, Ste. 4050
         Richmond, CA 94806                    San Francisco, CA  94111

                                      ALTA EMBARCADERO BIOPHARMA PARTNERS, LLC

                                      By: /s/ Eileen McCarthy
                                         -------------------------------
                                      Name:   Eileen McCarthy
                                           -----------------------------
                                      Title: Member
                                            ----------------------------

                                      Address: One Embarcadero Center, Ste. 4050
                                               San Francisco, CA  94111

                                      ONYX CHASE PARTNERS (ALTA BIO), LLC
                                      By: Alta/Chase BioPharma Management, LLC

                                      By: /s/ Eileen McCarthy
                                         -------------------------------
                                      Name:   Eileen McCarthy
                                           -----------------------------
                                      Title: Member
                                            ----------------------------

                                      Address: One Embarcadero Center, Ste. 4050
                                               San Francisco, CA  94111

<PAGE>

                                         DOMAIN PARTNERS IV, L.P.
                                         By: One Palmer Square Associates IV,
                                             L.L.C.
                                             Its General Partner

                                         By:   /s/ Kathleen K. Shoemaker
                                             ----------------------------------
                                                   Kathleen K. Shoemaker
                                                   Managing Member

                                         Address:  One Palmer Square
                                                   Princeton, NJ  08542

                                         DP IV ASSOCIATES, L.P.
                                         By: One Palmer Square Associates IV,
                                             L.L.C.
                                             Its General Partner

                                         By:   /s/ Kathleen K. Shoemaker
                                             ----------------------------------
                                                   Kathleen K. Shoemaker
                                                   Managing Member

                                         Address:  One Palmer Square
                                                   Princeton, NJ  08542

                                         INTERNATIONAL BIOTECHNOLOGY TRUST PLC

                                         By: /s/ Jeremy L. Curnock Cook
                                             ----------------------------------
                                         Name: Jeremy L. Curnock Cook
                                               --------------------------------
                                         Title:    Director
                                               --------------------------------
                                         Address:  Five Arrows House
                                                   St. Swithin's Lane
                                                   London EC4N 8 NR
                                                   England

<PAGE>

                                    EXHIBIT A

                              SCHEDULE OF PURCHASES

<TABLE>
<CAPTION>
          NAME                               SHARES        PURCHASE PRICE
<S>                                        <C>            <C>
Alta BioPharma Partners, L.P.                690,651          $6,215,859
Alta Embarcadero BioPharma, LLC               26,032             234,288
ONYX Chase Partners (Alta Bio), LLC          394,428           3,549,852
International Biotechnology Trust plc        222,222           1,999,998
Domain Partners IV, L.P.                     651,065           5,859,585
DP IV Associates, L.P.                        15,602             140,418
                                           ---------         -----------
         Total                             2,000,000         $18,000,000
</TABLE>

Source: [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00002-of-00352.parquet"}, [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00002-of-00352.parquet"}]]