<PAGE>
                                                                   EXHIBIT 10.28


                           ONYX PHARMACEUTICALS, INC.

                            STOCK PURCHASE AGREEMENT

                                JANUARY 18, 2000


<PAGE>

                                TABLE OF CONTENTS

<TABLE>
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                                                                                   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

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                                TABLE OF CONTENTS
                                   (CONTINUED)

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                                                                                   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>