Document:

<PAGE>   1
                                                                   EXHIBIT 10.73

                              [EPIMMUNE LETTERHEAD]

October 6, 1999

Stephen F. Keane
2780 Angell Avenue
San Diego, California 92122

Dear Steve,

This letter defines the benefits to which you will be entitled upon termination
of your employment with Epimmune Inc. (the "Company") under the circumstances
described in this Agreement.

We agree that you are employed by the Company as an "at-will" employee and that
either you or the Company has the right at any time to terminate your employment
with the Company, with or without cause or advance notice, for any reason or for
no reason.

For purposes of this Agreement, the following terms will have the meanings set
forth below:

        "BASE SALARY" means your salary (excluding bonus, any other incentive or
        other payments and stock option exercises) paid by the Company in
        consideration for your service during the 12 months prior to the
        termination of your employment which is includable in your gross income
        for federal income tax purposes or which would have been includable in
        gross income except for an election either under Section 125 or
        402(e)(3) of the Internal Revenue Code of 1986, as amended (the "Code"),
        or under the terms of a nonqualified deferred compensation arrangement
        sponsored by the Company.

        "BENEFIT" means any payment received or to be received by you pursuant
        to this Agreement.

        "CAUSE" means (i) your material breach of any provision of this
        Agreement; or (ii) your engaging or in any manner participating in any
        activity which is directly competitive with or intentionally injurious
        to the Company or which violates any provision of any confidential
        information agreement or nonsolicitation agreement between you and the
        Company; or (iii) your commission any fraud against the Company; or (iv)
        your intentional improper use or appropriation for your personal use or
        benefit of any funds or properties of the Company not authorized by the
        Board of Directors of the Company or an authorized committee of such
        Board of Directors to be so used or appropriated; or (v) your conviction
        of any crime involving dishonesty or moral turpitude.

<PAGE>   2
Stephen F. Keane
October 6, 1999
Page 2

        "CHANGE IN CONTROL" of the Company will be deemed to have occurred if
        and when (i) any person or entity (other than the Company) or group of
        persons and/or entities acting in concert acquires, directly or
        indirectly, beneficial ownership of more than 50% of the outstanding
        shares of voting stock of the Company; or (ii) the Company is a
        participant in a merger or consolidation in which it does not survive as
        an independent company; or (iii) the business or businesses of the
        Company for which your services are principally performed are disposed
        of by the Company pursuant to a partial or complete liquidation of the
        Company.

        "COMPANY" for the purposes of this Agreement shall include the Company's
        successor if a Change in Control occurs.

        "EXCISE TAX" means the excise tax imposed by Section 4999 of the Code.

        "GOOD REASON" means (i) reduction of your rate of compensation as in
        effect immediately prior to the effective date of this Agreement; or
        (ii) the Company's failure to provide to you a package of welfare
        benefit plans which, taken as a whole, provide substantially similar
        benefits to those in which you are entitled to participate immediately
        prior to the occurrence of the termination of your employment (except
        that employee contributions may be raised to the extent of any cost
        increases imposed by third parties) or any action by the Company which
        would adversely affect your participation or reduce your benefits under
        any of such plans; or (iii) a change in your responsibilities,
        authority, title or office resulting in diminution of your position,
        excluding for this purpose an isolated, insubstantial and inadvertent
        action not taken in bad faith which is remedied by the Company promptly
        after written notice thereof is given by you; or (iv) a request that you
        relocate to a worksite that is more than 35 miles from your prior
        worksite, unless you accept such relocation opportunity; or (v) a
        material reduction in your duties; or (vi) a failure or refusal of a
        successor to the Company to assume the Company's obligations under this
        Agreement; or (vii) a material breach by the Company or any successor to
        the Company of any of the material provisions of this Agreement.

Although the Company has no policy or procedure for providing severance
benefits, in exchange for the promises and covenants set forth herein, and in
consideration thereof, the Company agrees that if, within 365 days following the
occurrence of a Change in Control, your employment is terminated without Cause
by the Company or you terminate your employment for Good Reason, then upon your
furnishing to the Company an executed waiver and release of claims (a form of
which is attached hereto as Exhibit A) ("Release") and subject to your ongoing
obligations under the Company's Proprietary Information and Inventions
Agreement, (1) within 30 days of the Effective Date of the Release you will
receive a lump-sum payment equal to six

<PAGE>   3
Stephen F. Keane
October 6, 1999
Page 3

months of Base Salary subject to any applicable withholding of federal, state or
local taxes and (2) the vesting of any stock option granted to you for Company
common stock and the time during which such stock option may be exercised
immediately will be accelerated as to 50% of the then unvested stock options,
such that you will have the right to immediately exercise such stock options on
the date of termination.

If any Benefit received or to be received by you pursuant to this Agreement
would constitute a "parachute payment" within the meaning of Section 280G of the
Code and but for this paragraph be subject to the Excise Tax, then, upon written
request to the Commune by you, such Benefit shall be reduced to the extent
necessary so that no portion of the Benefit would be subject to the Excise Tax,
as determined in good faith by the Company; provided, however, that if, in the
absence of any such reduction (or after such reduction), you believe that the
Benefit or any portion thereof (as reduced, if applicable) would be subject to
the Excise Tax, the Benefit shall be reduced (or further reduced) to the extent
determined by you in your discretion so that the Excise Tax would not apply.
Your determination of any required reduction pursuant to this paragraph shall be
conclusive and binding on the Company. If, notwithstanding any such reduction
(or in the absence of such reduction), the Internal Revenue Service determines
that you are liable for the Excise Tax as a result of the Benefit, you shall be
obligated to return to the Company, within 30 days of such determination by the
Internal Revenue Service, a portion of the Benefit sufficient such that none of
the Benefit retained by you constitutes a "parachute payment" within the meaning
of Code Section 280G that is subject to the Excise Tax (the "Repayment Amount").
The Repayment Amount shall be the smallest such amount, if any, as shall be
required to be paid to the Company so that your net proceeds with respect to the
Benefit (after taking into account the payment of the Excise Tax imposed on such
Benefit) shall be maximized. Notwithstanding the foregoing, the Repayment Amount
with respect to the Benefit shall be zero if a Repayment Amount of more than
zero would not eliminate the Excise Tax imposed on such Benefit. If the Excise
Tax is not eliminated pursuant to your compliance with this paragraph, you agree
to pay the Excise Tax.

This Agreement may be changed or terminated only upon the mutual written consent
of the Company and you. Nothing in this Agreement will prevent or limit your
continuing or future participation in any benefit, bonus, incentive or other
plans, programs, policies or practices provided by the Company and for which you
may otherwise qualify, nor will anything herein limit or otherwise affect such
rights as you may have under any stock option or other agreements with the
Company.

If either party hereto brings any action to enforce such party's rights
hereunder, the prevailing party in any such action will be entitled to recover
such party's reasonable attorneys' fees and costs incurred in connection with
such action.

<PAGE>   4
This Agreement constitutes the entire agreement between you and the Company
regarding the subject matter herein. It is entered into without reliance on any
promise or representation other than those expressly contained herein. All
questions concerning the construction, validity and interpretation of this
Agreement will be governed by the law of the State of California.

Very truly yours,

Deborah A. Schueren
President and CEO

DAS/dw

Agreed and Accepted:

/s/ Stephen F. Keane
-------------------------------
Stephen F. Keane

Dated:  October 8, 1999

<PAGE>   5
                                    EXHIBIT A
                          RELEASE AND WAIVER OF CLAIMS

        In exchange for consideration provided to me in the Agreement to which
this form is attached, I hereby furnish Epimmune Inc. (the "Company") with the
following release and waiver.

        I hereby release, and forever discharge the Company, its officers,
directors, agents, employees, stockholders, successors, assigns and affiliates,
of and from any and all claims, liabilities, demands, causes of action, costs,
expenses, attorneys fees, damages, indemnities and obligations of every kind and
nature, in law, equity, or otherwise, known and unknown, suspected and
unsuspected, disclosed and undisclosed, arising at any time prior to and
including my employment termination date with respect to any claims relating to
my employment and the termination of my employment, including but not limited
to, claims pursuant to any federal, state or local law relating to employment,
including, but not limited to, discrimination claims, claims under the
California Fair Employment and Housing Act, and the Federal Age Discrimination
in Employment Act of 1967, as amended (ADEA), or claims for wrongful
termination, breach of the covenant of good faith, contract claims, tort claims,
and wage or benefit claims, including but not limited to, claims for salary,
bonuses, commissions, stock, stock options, vacation pay, fringe benefits,
severance pay or any form of compensation (other than the obligations under the
Agreement.)

        I also acknowledge that I have read and understand Section 1542 of the
California Civil Code which reads as follows: "A general release does not extend
to claims which the creditor does not know or suspect to exist in his favor at
the time of executing the release, which if known by him must have materially
affected his settlement with the debtor." I hereby expressly waive and
relinquish all rights and benefits under that section and any law of any
jurisdiction of similar effect with respect to any claims I may have against the
Company.

        I acknowledge that, among other rights, I am waiving and releasing any
rights I may have under ADEA, that this waiver and release is knowing and
voluntary, and that the consideration given for this waiver and release is in
addition to anything of value to which I was already entitled as an employee of
the Company. I further acknowledge that I have been advised, as required by the
Older Workers Benefit Protection Act, that: (a) the waiver and release granted
herein does not relate to claims which may arise after this agreement is
executed; (b) I have the right to consult with an attorney prior to executing
this agreement (although I may choose voluntarily not to do so); (c) I have
twenty-one (21) days from the date I receive this agreement in which to consider
this agreement (although I may choose voluntarily to execute this agreement
earlier); (d) I have seven (7) days following the execution of this agreement to
revoke my consent to the agreement; and (e) this agreement shall not be
effective until the seven (7) day revocation period has expired.

Dated:                                  By:
      -------------------------------      ---------------------------------<PAGE>   1
                                                                   EXHIBIT 10.74

                                  EPIMMUNE INC.

                                  COMMON STOCK
                               PURCHASE AGREEMENT

                               FEBRUARY ___, 2000

<PAGE>   2
                               TABLE OF CONTENTS

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

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

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

        2.2    Separate Agreements..........................................................        1

        2.3    Acceptance of Proposed Purchase of Shares....................................        1

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

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

        3.2    Delivery of the Shares at the Closing........................................        2

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

        4.1    Organization and Standing....................................................        2

        4.2    Corporate Power; Authorization...............................................        2

        4.3    Issuance and Delivery of the Shares..........................................        3

        4.4    SEC Documents; Financial Statements..........................................        3

        4.5    Intellectual Property........................................................        3

        4.6    Capitalization...............................................................        3

        4.7    Litigation...................................................................        4

        4.8    Governmental Consents........................................................        4

        4.9    No Material Adverse Change...................................................        4

5.      REPRESENTATIONS, WARRANTIES AND COVENANTS OF THE PURCHASER..........................        4

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

7.      CONDITIONS TO COMPANY'S OBLIGATIONS AT THE CLOSING..................................        6

        7.1    Receipt of Payment...........................................................        6

        7.2    Representations and Warranties Correct.......................................        6

8.      CONDITIONS TO PURCHASERS' OBLIGATIONS AT THE CLOSING................................        6

        8.1    Representations and Warranties Correct.......................................        6

        8.2    Legal Opinion................................................................        6

9.      REGISTRATION OF THE SHARES; COMPLIANCE WITH THE SECURITIES ACT......................        6

        9.1    Registration Procedures and Expenses.........................................        6

        9.2    Transfer of Securities After Registration....................................        8

        9.3    Legends......................................................................        8
</TABLE>

                                       i.

<PAGE>   3
                               TABLE OF CONTENTS
                                  (CONTINUED)

<TABLE>
<CAPTION>
                                                                                                  PAGE
<S>     <C>                                                                                       <C>
        9.4    Indemnification..............................................................        9

        9.5    Termination of Conditions and Obligations....................................       10

        9.6    Information Available........................................................       10

        9.7    Changes in Purchaser Information.............................................       11

10.     BROKER'S FEE........................................................................       11

11.     NOTICES.............................................................................       11

12.     MISCELLANEOUS.......................................................................       11

        12.1   Waivers and Amendments.......................................................       11

        12.2   Headings.....................................................................       12

        12.3   Severability.................................................................       12

        12.4   Governing Law................................................................       12

        12.5   Counterparts.................................................................       12

        12.6   Successors and Assigns.......................................................       12

        12.7   Entire Agreement.............................................................       12

        12.8   Payment of Fees and Expenses.................................................       12
</TABLE>

                                      ii.

<PAGE>   4
                                  COMMON STOCK
                               PURCHASE AGREEMENT

        THIS AGREEMENT ("Agreement") is made as of February ___, 2000 (the
"Effective Date"), by and between Epimmune Inc., a Delaware corporation with its
principal place of business at 5820 Nancy Ridge Drive, Suite 100, San Diego,
California 92121 (the "Company"), and the purchaser whose name and address is
set forth on the signature page hereof (the "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 the Purchaser hereby agree as follows:

        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 ____________
shares of its Common Stock (the "Common Stock") pursuant to the Agreements (as
defined below). The shares of Common Stock sold to the Purchaser hereunder shall
be referred to herein as the "Shares."

        2.      AGREEMENT TO SELL AND PURCHASE THE SHARES.

               2.1 SALE OF SHARES. At the Closing (as defined in Section 3), the
Company will sell to the Purchaser, and the Purchaser will purchase from the
Company, at a purchase price of $3.60 per Share, the number of Shares set forth
below:

<TABLE>
<CAPTION>
                     NUMBER OF SHARES                 AGGREGATE
                      TO BE PURCHASED               PURCHASE PRICE
                      ---------------               --------------
<S>                                                 <C>

</TABLE>

               2.2 SEPARATE AGREEMENTS. The Company proposes to enter the same
form of purchase agreement with certain other investors (the "Other Purchasers")
and expects to complete sales of Shares to them. The Purchaser and the Other
Purchasers are hereinafter sometimes collectively referred to as the
"Purchasers," and this Agreement and the agreements executed by the Other
Purchasers are hereinafter sometimes collectively referred to as the
"Agreements."

               2.3 ACCEPTANCE OF PROPOSED PURCHASE OF SHARES. The Company shall
have no obligation hereunder with respect to the Purchaser until the Company
shall execute and deliver to the Purchaser an executed copy of this Agreement.
If this Agreement is not executed and delivered by the Company, this Agreement
shall be of no further force and effect.

        3.      CLOSING AND DELIVERY.

               3.1 CLOSING. The Closing of the purchase and sale of the Shares
pursuant to this Agreement (the "Closing") shall be held as soon as practicable
after the satisfaction or

                                       1.

<PAGE>   5
waiver of all conditions to Closing set forth in Sections 7 and 8 hereof, at the
offices of Cooley Godward LLP, 4365 Executive Drive, Suite 1100, California
92121, or on such other date and place as may be agreed to by the Company and
the Purchaser. At or prior to the Closing, the Purchaser shall execute any
related agreements or other documents required to be executed hereunder, dated
as of the date of the Closing (the "Closing Date").

               3.2 DELIVERY OF THE SHARES AT THE CLOSING. At the Closing, the
Company shall deliver to the Purchaser stock certificates registered in the name
of the Purchaser, and/or in such nominee name(s) as designated by the Purchaser,
representing the number of Shares to be purchased by the Purchaser at the
Closing as set forth in Section 2.1 against payment of the purchase price for
such Shares. The name(s) in which the stock certificates are to be issued to the
Purchaser are set forth in the Questionnaire in the form attached hereto as
Appendix I, as completed by the Purchaser.

        4.      REPRESENTATIONS, WARRANTIES AND COVENANTS OF THE COMPANY.

        Except as set forth on the Schedule of Exceptions attached hereto as
Exhibit A, the Company hereby represents and warrants as of the date hereof to,
and covenants with, the Purchaser as follows:

               4.1 ORGANIZATION AND STANDING. The Company has been duly
incorporated and is validly existing as a corporation in good standing under the
laws of the State of Delaware, has full corporate power and authority to own or
lease its properties and conduct its business as presently conducted (as defined
below), and is duly qualified as a foreign corporation and in good standing in
all jurisdictions in which the character of the property owned or leased or the
nature of the business transacted by it makes qualification necessary (except
where the failure to be so qualified would not have a material adverse effect on
the business, properties, financial condition or results or operations of the
Company).

               4.2 CORPORATE POWER; AUTHORIZATION. The Company has all requisite
corporate power, and has taken all requisite corporate action, to execute and
deliver this Agreement, sell and issue the Shares and carry out and perform all
of its obligations under this Agreement. This Agreement constitutes the legal,
valid and binding obligation of the Company, enforceable in accordance with its
terms, except (i) as limited by applicable bankruptcy, insolvency,
reorganization, moratorium or similar laws relating to or affecting the
enforcement of creditors' rights generally, (ii) as limited by equitable
principles generally, including any specific performance, and (iii) as to those
provisions of Section 9.4 relating to indemnity or contribution. The execution
and delivery of this Agreement does not, and the performance of this Agreement
and the compliance with the provisions hereof and the issuance, sale and
delivery of the Shares by the Company will not, conflict with, or result in a
breach or violation of the terms, conditions or provisions of, or constitute a
default under, or result in the creation or imposition of any lien pursuant to
the terms of, the Certificate of Incorporation or Bylaws of the Company or any
statute, law, rule or regulation applicable to the Company or any state or
federal order, judgment or decree applicable to the Company or any indenture,
mortgage, lease or other agreement or instrument to which the Company or any of
its properties is subject, where such conflict, breach or violation would have a
material adverse effect on the Company.

                                       2.

<PAGE>   6
               4.3 ISSUANCE AND DELIVERY OF THE SHARES. The Shares, when issued
and paid for in compliance with the provisions of this Agreement, will be
validly issued, fully paid and nonassessable and will not be issued in violation
of any preemptive right.

               4.4 SEC DOCUMENTS; FINANCIAL STATEMENTS. Each complete or partial
statement or report included as an appendix to the SEC Documents is a true and
complete copy of or excerpt from such document as filed by the Company with the
United States Securities and Exchange Commission (the "SEC"). The Company has
filed in a timely manner all documents that the Company was required to file
with the SEC under Sections 13, 14(a) and 15(d) of the Securities Exchange Act
of 1934, as amended (the "Exchange Act"), during the twelve (12) months
preceding the date of this Agreement. As of their respective filing dates (or,
if amended, when amended), all documents filed by the Company with the SEC (the
"SEC Documents") complied in all material respects with the requirements of the
Exchange Act. None of the SEC Documents as of their respective dates contained
any untrue statement of material fact or omitted to state a material fact
required (under the federal securities laws in connection with the sale of the
Shares) to be stated therein or necessary to make the statements made therein,
in light of the circumstances under which they were made, not misleading. The
financial statements of the Company included in the SEC Documents (the
"Financial Statements") comply as to form in all material respects with
applicable accounting requirements and with the published rules and regulations
of the SEC with respect thereto. The Financial Statements have been prepared in
accordance with generally accepted accounting principles consistently applied
and fairly present the financial position of the Company at the dates thereof
and the results of its operations and cash flows for the periods then ended
(subject, in the case of unaudited statements, to normal, recurring
adjustments).

               4.5 INTELLECTUAL PROPERTY. The Company owns or possesses adequate
rights to use all material patents, patent rights, inventions, trade secrets and
know-how that are necessary for the conduct of its business as presently
conducted and as described in the SEC Documents. Except as set forth in the SEC
Documents, the Company has not received any notice of, nor has any knowledge of,
any infringement of or conflict with asserted rights of others with respect to
any patent, patent right, invention, trade secret or know-how that, individually
or in the aggregate, if the subject of an unfavorable decision, ruling or
finding, would have a material adverse effect on the business, properties,
financial condition or results or operations of the Company.

               4.6 CAPITALIZATION. The capitalization of the Company is
described in the Company's SEC Documents. The Company has not issued any capital
stock since September 30, 1999 other than pursuant to employee benefit plans
disclosed in the Company's SEC Documents. The Shares to be sold pursuant to this
Agreement have been duly authorized, and when issued and paid for in accordance
with the terms of this Agreement, will be duly and validly issued, fully paid
and nonassessable. The outstanding shares of capital stock of the Company have
been duly and validly issued and are fully paid and nonassessable, have been
issued in compliance with all federal and state securities laws, and were not
issued in violation of any preemptive rights or similar rights to subscribe for
or purchase securities. Except as set forth in or contemplated by the Company's
SEC Documents or as otherwise described in this Agreement, there are no
outstanding rights (including, without limitation, preemptive rights), warrants
or options to acquire, or instruments convertible into or exchangeable for, any
unissued

                                       3.

<PAGE>   7
shares of capital stock or other equity interest in the Company, or any
contract, commitment, agreement, understanding or arrangement of any kind to
which the Company is a party and relating to the issuance or sale of any capital
stock of the Company, any such convertible or exchangeable securities or any
such rights, warrants or options.

               4.7 LITIGATION. There is no pending or, to the Company's
knowledge, threatened, action, suit or other proceeding to which the Company is
a party or to which its property or assets are subject that is not disclosed in
the SEC Documents that is required to be so disclosed.

               4.8 GOVERNMENTAL CONSENTS. No consent, approval, order or
authorization of, or registration, qualification, designation, declaration or
filing with, any federal, state, or local governmental authority on the part of
the Company is required in connection with the consummation of the transactions
contemplated by this Agreement except for (a) compliance with the securities and
blue sky laws in the states and other jurisdictions in which shares of Common
Stock are offered and/or sold, which compliance will be effected in accordance
with such laws, and (b) the filing of a registration statement and all
amendments thereto with the SEC as contemplated by Section 9.1 of this
Agreement.

               4.9 NO MATERIAL ADVERSE CHANGE. Except as otherwise disclosed
herein, since September 30, 1999, there have not been any changes in the assets,
liabilities, financial condition or operations of the Company from that
reflected in the Company's Form 10-Q for the period ended September 30, 1999
except changes in the ordinary course of business or which have not been, either
individually or in the aggregate, materially adverse.

               4.10 SECURITIES VIOLATIONS. The Company represents and warrants
that none of its directors or officers is or has within the last five years,
been the subject of, or a defendant in: (i) an enforcement action or prosecution
(or settlement in lieu thereof) brought by a governmental authority relating to
a violation of securities, tax, fiduciary or criminal laws, or (ii) a civil
action (or settlement in lieu thereof) brought by shareholders or investors in a
common investment vehicle for violation of duties owed to the shareholders or
investors.

               4.11 NASDAQ. The Company's Common Stock is listed on the Nasdaq
National Market and the Company shall use its commercially reasonable efforts to
maintain such listing.

               4.12 NO CONFLICTS OF INTEREST REPRESENTATION. The Company
represents, warrants and covenants that, to the best of its knowledge, no
officer or employee of the Purchaser has or will receive, directly or
indirectly, a personal interest in the Company or its property or anything of
substantial economic value for his or her private benefit from the Company, or
anyone acting on its behalf, in connection with the investment made by the
Purchaser pursuant to this Agreement.

               4.13 FUTURE ISSUANCES OF COMMON STOCK. The Company covenants that
during the ninety (90) days following the Closing it will not issue additional
shares of Common Stock at a price per share of less than $3.60, other than stock
issued upon exercise of (i) outstanding warrants to purchase the Company's
Common Stock, (ii) warrants to purchase the Company's Common Stock to be issued
to certain former executive officers of the Company in

                                       4.

<PAGE>   8
connection with severance agreements with such individuals, (iii) options to
purchase the Company's Common Stock issued to employees, directors and
consultants of the Company.

        5.      REPRESENTATIONS, WARRANTIES AND COVENANTS OF THE PURCHASER.

               5.1 The Purchaser represents and warrants to and covenants with
the Company that:

                      (a) The Purchaser, taking into account the personnel and
resources it can practically bring to bear on the purchase of the Shares
contemplated hereby, either alone or together with the advice of the Purchaser's
representative, is knowledgeable, sophisticated and experienced in making, and
is qualified to make, decisions with respect to investments in shares presenting
an investment decision like that involved in the purchase of the Shares,
including investments in securities issued by the Company, and has requested,
received, reviewed and considered, either alone or with the Purchaser's
representative, all information the Purchaser deems relevant (including the SEC
Documents) in making an informed decision to purchase the Shares.

                      (b) The Purchaser is acquiring the Shares being acquired
by the Purchaser pursuant to this Agreement in the ordinary course of its
business and for its own account for investment only and with no present
intention of distributing any of such Shares or any arrangement or understanding
with any other persons regarding the distribution of such Shares, except in
compliance with Section 5.1(c).

                      (c) The Purchaser will not, directly or indirectly, offer,
sell, pledge, transfer or otherwise dispose of (or solicit any offers to buy,
purchase or otherwise acquire or take a pledge of) any of the securities
purchased hereunder except in compliance with the Securities Act of 1933, as
amended (the "Securities Act"), applicable blue sky laws, and the rules and
regulations promulgated thereunder.

                      (d) The Purchaser has completed or caused to be completed
the subscription documents enclosed with this Agreement, including the
Registration Questionnaire, for use in preparation of the Registration Statement
to be filed by the Company, and the answers thereto are true and correct as of
the date hereof and will be true and correct as of the effective date of the
applicable Registration Statement (provided that the Purchaser shall be entitled
to update such information by providing notice thereof to the Company prior to
the effective date of such Registration Statement).

                      (e) The Purchaser has, in connection with its decision to
purchase the Securities, relied with respect to the Company and its affairs
solely upon the SEC Documents and the other information delivered to the
Purchaser by the Company as described in Sections 4.4 and 5.1(a) above and the
representations and warranties of the Company contained herein.

                      (f) The Purchaser is an "accredited investor" within the
meaning of Rule 501 of Regulation D promulgated under the Securities Act or a
Qualified Institutional Buyer within the meaning of Rule 144A promulgated under
the Securities Act.

                                       5.

<PAGE>   9
                      (g) The Purchaser has full right, power, authority and
capacity to enter into this Agreement and to consummate the transactions
contemplated hereby and has taken all necessary action to authorize the
execution, delivery and performance of this Agreement. Upon the execution and
delivery of this Agreement by the Purchaser, this Agreement shall constitute a
valid and binding obligation of the Purchaser, enforceable in accordance with
its terms, except (i) as limited by applicable bankruptcy, insolvency,
reorganization, moratorium or similar laws relating to or affecting the
enforcement of creditors' rights generally, (ii) as limited by equitable
principles generally, including any specific performance, and (iii) as to those
provisions of Section 9.4 relating to indemnity or contribution.

               5.2 The Purchaser represents and warrants to and covenants with
the Company that it has not engaged and will not engage in any short sales of
the Company's Common Stock prior to the effectiveness of the Registration
Statement and for so long as such Purchaser holds such Shares.

               5.3 The Purchaser understands that nothing in the SEC Documents,
this Agreement or any other materials presented to the Purchaser in connection
with the purchase and sale of the Shares constitutes legal, tax or investment
advice and that no independent legal counsel has reviewed these documents and
materials on the Purchaser's behalf. The Purchaser has consulted such legal, tax
and investment advisors as it, in its sole discretion, has deemed necessary or
appropriate in connection with its purchase of the Shares.

        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
the Purchaser herein and in the certificates for the securities delivered
pursuant hereto shall survive the execution of this Agreement, the delivery to
the Purchaser of the Shares being purchased and the payment therefor.

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

               7.1 RECEIPT OF PAYMENT. The Company shall have received payment,
by check or wire transfer of immediately available funds, in the full amount of
the purchase price for the number of Shares being purchased by the Purchaser at
the Closing as set forth in Section 2.1.

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

        8.      CONDITIONS TO PURCHASERS' OBLIGATIONS AT THE CLOSING. The
Purchaser's obligation to accept delivery of the Shares and to pay for the
Shares shall be subject to the following conditions to the extent not waived by
such Purchaser:

               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. At

                                       6.

<PAGE>   10
the Closing, the President of the Company shall deliver a certificate certifying
that the representations and warranties made by the Company in Section 4 are
true and correct as of the date of the Closing.

               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.

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

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

                      (a) As soon as practicable following the Closing and in
any event no later than fifteen (15) business days following the Closing, the
Company shall prepare and file with the SEC one or more registration statements
in order to register with the SEC the resale by the Purchasers, from time to
time, of the Shares through Nasdaq 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"). The Company shall use its
best efforts to cause such Registration Statement to be declared effective as
soon thereafter as reasonably possible. The Company shall promptly notify the
Purchaser of the effectiveness of such Registration Statement.

                      (b) The Company shall prepare and file with the SEC (i)
such amendments and supplements to the Registration Statement and the prospectus
used in connection therewith, (ii) such SEC Documents and (iii) such other
filings required by the SEC, in each case as may be necessary to keep the
Registration Statement continuously effective and not misleading until the
earliest of (A) the second anniversary date of the Closing, (B) such date as all
of the Shares held by all of the Purchasers have been resold or (C) such time as
all of the Shares held by the Purchasers can be sold within a given three-month
period pursuant to Rule 144 under the Securities Act. Notwithstanding the
foregoing, if, at any time following the effectiveness of the Registration
Statement, the Company shall have determined that the Company may be required to
disclose any material corporate development, the Company may suspend the
effectiveness 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 Exchange Act, which suspension shall endure for
such period as deemed necessary by the Company upon advice of counsel (a
"Suspension Period"), by giving notice to the Purchasers. The Company will use
commercially reasonable efforts to limit the length of any Suspension Periods to
less than 60 days in any twelve month period. The Purchaser agrees that, upon
receipt of any notice from the Company of a Suspension Period, the Purchaser
will not sell any Shares pursuant to the Registration Statement until (i) the
Purchaser is advised in writing by the Company that the use of the applicable
prospectus may be resumed, (ii) the Purchaser has received copies of any
additional or supplemental or amended prospectus, if applicable, and (iii) the
Purchaser has received copies of any additional or supplemental filings which
are incorporated or deemed to be incorporated by reference in such prospectus.

                                       7.

<PAGE>   11
                      (c) In order to facilitate the public sale or other
disposition of all or any of the shares by the Purchaser, the Company shall
furnish to the Purchaser with respect to the Shares registered under the
Registration Statement such number of copies of prospectuses, prospectus
supplements and preliminary prospectuses as the Purchaser reasonably requests in
conformity with the requirements of the Securities Act.

                      (d) The Company shall file any documents required of the
Company for normal blue sky clearance in states specified in writing by the
Purchaser; provided, however, that the Company shall not be required to qualify
to do business or consent to service of process in any jurisdiction in which it
is not now so qualified or has not so consented.

                      (e) Other than fees and expenses, if any, of counsel or
other advisers to the Purchasers, which fees and expenses shall be borne by the
Purchasers except as referred to in Section 12.8 below, the Company shall bear
all expenses (exclusive of any brokerage fees, underwriting discounts and
commissions) in connection with the procedures in paragraphs (a) through (d) of
this Section 9.1 and if requested by holders of at least a majority of the
Shares, the fees and expenses, if any, of one counsel to the Selling
Stockholders (as defined in Section 9.4) in an amount not exceeding $10,000.

                      (f) With a view to making available to the Purchasers the
benefits of Rule 144 promulgated under the Securities Act ("Rule 144") and any
other rule or regulation of the SEC that may at any time permit a Purchaser to
sell Shares to the public without registration or pursuant to registration, the
Company covenants and agrees to: (i) make and keep public information available,
as those terms are understood and defined in Rule 144, until the earlier of (A)
the second anniversary of the Closing Date or (B) such date as all of the Shares
shall have been resold; (ii) file with the SEC in a timely manner all reports
and other documents required of the Company under the Exchange Act; and (iii)
furnish to any Purchaser upon request, as long as the Purchaser owns any Shares,
(A) a written statement by the Company that it has complied with the reporting
requirements of the Exchange Act, (B) a copy of the most recent annual or
quarterly report of the Company, and (C) such other information as may be
reasonably requested in order to avail any Purchaser of any rule or regulation
of the SEC that permits the selling of any such Shares without registration
under the Securities Act.

               9.2 TRANSFER OF SECURITIES AFTER REGISTRATION. The Purchaser
agrees that it will not effect any disposition of the Shares that would
constitute a sale within the meaning of the Securities Act, except:

                      (a) pursuant to the Registration Statement, in which case
the Purchaser shall submit the certificates evidencing the Shares to the
Company's transfer agent, accompanied by a separate "Purchaser's Certificate"
(i) in the form of Appendix II attached hereto, (ii) executed by such Purchaser
or by an officer of, or other authorized person designated by, such Purchaser,
and (iii) to the effect that (A) the Shares have been sold in accordance with
the Registration Statement and (B) the requirement of delivering a current
prospectus has been satisfied; or

                      (b) the Purchaser shall have notified the Company of the
proposed disposition and shall have furnished the Company with a detailed
statement of the circumstances

                                       8.

<PAGE>   12
surrounding the proposed disposition, and if reasonably requested by the
Company, the Purchaser shall have furnished the Company with an opinion of
counsel, reasonably satisfactory to the Company, that such disposition will not
require registration of such Shares under the Securities Act. It is agreed that
the Company will not require opinions of counsel for transactions made pursuant
to Rule 144 except in unusual circumstances.

               9.3 LEGENDS. Each certificate representing Shares shall (unless
otherwise permitted by the provisions of the Agreement) be stamped or otherwise
imprinted with a legend substantially similar to the following (in addition to
any legend required under applicable state securities laws or as provided
elsewhere in this Agreement):

                THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN
        REGISTERED UNDER THE SECURITIES ACT OF 1933 (THE "ACT") AND MAY NOT BE
        OFFERED, SOLD OR OTHERWISE TRANSFERRED, PLEDGED OR HYPOTHECATED UNLESS
        AND UNTIL REGISTERED UNDER THE ACT OR, IN THE OPINION OF COUNSEL OR
        BASED ON OTHER WRITTEN EVIDENCE IN THE FORM AND SUBSTANCE SATISFACTORY
        TO THE ISSUER OF THESE SECURITIES, SUCH OFFER, SALE OR TRANSFER, PLEDGE
        OR HYPOTHECATION IS IN COMPLIANCE THEREWITH.

               9.4 INDEMNIFICATION. As used in this Section 9.4 the following
terms shall have the following respective meanings:

                      (a) "SELLING STOCKHOLDER" shall mean the Purchaser and any
transferee of the Purchaser who is entitled to resell Shares pursuant to the
Registration Statement;

                      (b) "REGISTRATION STATEMENT" shall include any final
prospectus, exhibit, supplement or amendment included in or relating to the
Registration Statement referred to in Section 9.1; and

                      (c) "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.

The Company agrees to indemnify and hold harmless each Selling Stockholder from
and against any losses, claims, damages or liabilities to which such Selling
Stockholder 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 9.1 or 9.2 hereof, or any Untrue
Statement on or after the effective date of the Registration Statement, or on or
after the date of any prospectus or prospectus supplement or the date of any
sale by Purchaser thereunder, or arise out of any failure by the Company to
fulfill any undertaking included in the Registration Statement and the Company
will reimburse such Selling Stockholder 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

                                       9.

<PAGE>   13
be liable to such Selling Stockholder 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
Selling Stockholder specifically for use in preparation of the Registration
Statement, or the failure of such Selling Stockholder to comply with the
covenants and agreements contained in Section 9.1 or 9.2 hereof respecting sale
of the Shares or any statement or omission in any Prospectus that is corrected
in any subsequent prospectus that was delivered to the Selling Stockholder prior
to the pertinent sale or sales by the Selling Stockholder.

        The Purchaser agrees to indemnify and hold harmless the Company (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 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 9.1 or 9.2 hereof
respecting sale of the Shares, or any Untrue Statement contained in the
Registration Statement on or after the effective date thereof, or in any
prospectus supplement as of its issue date or date of any sale by the Purchaser
thereunder, if such Untrue Statement was made in reliance upon and in conformity
with written information furnished by or on behalf of the Purchaser specifically
for use in preparation of the Registration Statement, and the Purchaser will
reimburse the Company (or such 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 the Purchaser under this
Section 9.4 exceed the gross proceeds received by the Purchaser from the sale of
Shares covered by such Registration Statement.

        Promptly after receipt by any indemnified person of a 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 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.

                                      10.

<PAGE>   14
               9.5 TERMINATION OF CONDITIONS AND OBLIGATIONS. The conditions
precedent imposed by Section 4, Section 5 or this Section 9 upon the
transferability of the Shares shall cease and terminate as to any particular
number of the Shares when such Shares shall have been sold or otherwise disposed
of in accordance with the intended method of disposition set forth in the
Registration Statement covering such Shares or at such time as an opinion of
counsel satisfactory to the Company shall have been rendered to the effect that
such conditions are not necessary in order to comply with the Securities Act.

               9.6 INFORMATION AVAILABLE. So long as the Registration Statement
is effective covering the resale of Shares owned by the Purchaser, the Company
will furnish to the Purchaser:

                      (a) as soon as practicable after available (but in the
case of the Company's Annual Report to Stockholders, within 120 days after the
end of each fiscal year of the Company), one copy of (i) its Annual Report to
Stockholders (which Annual Report shall contain financial statements audited in
accordance with generally accepted auditing standards certified by a national
firm of certified public accountants); (ii) its Annual Report on Form 10-K;
(iii) its quarterly reports on Form 10-Q (the foregoing, in each case, excluding
exhibits); (iv) its Proxy Statement; and (v) its current reports on Form 8-K, if
any;

                      (b) upon the request of any Purchaser, all exhibits
excluded by the parenthetical in subparagraph (a)(iii) of this Section 9.6, in
the form generally available to the public; and

                      (c) upon the reasonable request of any Purchaser, an
adequate number of copies of the prospectuses and supplements to supply to any
other party requiring such prospectuses.

               9.7 CHANGES IN PURCHASER INFORMATION. The Purchaser agrees to
promptly notify the Company of any changes in the information set forth in the
Registration Statement regarding the Purchaser or its plan of distribution set
forth in such Registration Statement.

        10. BROKER'S FEE. The Company and the Purchaser hereby represent that,
there are no brokers or finders entitled to compensation in connection with the
sale of the Shares, and shall indemnify each other for any such fees for which
they are responsible.

        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 in the
case of facsimile transmission, or when so received in the case of mail or
courier, and addressed as follows:

               (a) if to the Company, to:

                   Epimmune Inc.
                   5820 Nancy Ridge Drive, Suite 100
                   San Diego, CA 92121
                   Attention:  President and Chief Executive Officer

                                      11.

<PAGE>   15
               with a copy so mailed to:

                   Cooley Godward LLP
                   4365 Executive Drive, Suite 1100
                   San Diego, CA 92121
                   Attention: Frederick T. Muto, 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 Purchaser, 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.

        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 the Purchaser.

               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 California as applied to
contracts entered into and performed entirely in California by California
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.

               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.

               12.7 ENTIRE AGREEMENT. This Agreement 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.

               12.8 PAYMENT OF FEES AND EXPENSES. Each of the Company and the
Purchaser shall bear its own expenses and legal fees incurred on its behalf with
respect to this Agreement

                                      12.

<PAGE>   16
and the transactions contemplated hereby (the "Offering"). If any action at law
or in equity is necessary to enforce or interpret the terms of this Agreement,
the prevailing party shall be entitled to reasonable attorney's fees, costs and
necessary disbursements in addition to any other relief to which such party may
be entitled.

                                      13.

<PAGE>   17
        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.

                                     EPIMMUNE INC.

                                     By:
                                        ------------------------------------
                                     Name:
                                          ----------------------------------

                                     Title:
                                           ---------------------------------

                                     PURCHASER

                                     Purchaser Name:
                                                    ------------------------

                                     By:
                                        ------------------------------------

                                     Name:
                                          ----------------------------------

                                     Title:
                                           ---------------------------------

                                     Address:
                                             -------------------------------

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

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

                                     Facsimile:
                                               -----------------------------

                        COMMON STOCK PURCHASE AGREEMENT

<PAGE>   18
                                                                      APPENDIX I

                              EPIMMUNE CORPORATION
                                  QUESTIONNAIRE

        In connection with the issuance of stock certificates and preparation of
the Registration Statement, please provide us with the following information:

        1.      Please state you or your organization's name exactly as it
                should appear on the stock certificate representing your Shares
                in the Registration Statement:

        2.      Please provide the following information, as of February ___,
                2000:

<TABLE>
<S>                                                 <C>
                 Number of Shares that you are              Number of shares of Common
         purchasing and seek to include in the      Stock that you already beneficially
         Registration Statement:                    own or that you are purchasing and do
                                                    NOT seek to include in the
                 __________________                 Registration Statement:

                                                            ______________________
</TABLE>

        3. Have you or your organization had any position, office or other
material relationship within the past three years with the Company or its
affiliates other than as disclosed in the Proxy Statement in connection with the
Company's 1999 Annual Meeting of Stockholders?

                                 Yes [ ] No [ ]

        If yes, please indicate the nature of any such relationships:___________
________________________________________________________________________________

                                            Signature:
                                                      ------------------------

                                            Print Name:
                                                       -----------------------

                                            Title:
                                                  ----------------------------

<PAGE>   19
                                                                     APPENDIX II

                 PURCHASER'S CERTIFICATE OF RESALE OF THE SHARES

        The undersigned, an officer of, or other person duly authorized by
                                                     hereby certifies that
----------------------------------------------------
[fill in official name of individual or institution]
he/she [said institution] is the Purchaser of the Shares evidenced by the
attached stock certificate(s) and as such, sold such Shares on
                               in accordance with registration statement number
-------------------------------
          [date]
                                                                      and the
--------------------------------------------------------------------
[fill in the number of or otherwise identify registration statement]
requirement of delivering a current prospectus and current annual, quarterly and
reports (Forms 10-K, 10-Q, and 8-K) by the Company has been complied with in
connection with such sale.

Print or Type:

Name of Purchaser (Individual or Institution):
                                              ---------------------------------

Name of Individual representing Purchaser
(if an Institution):
                                                  -----------------------------
Title of Individual representing Purchaser
(if an Institution):
                                                  -----------------------------

Signature by:

Individual Purchaser or Individual
representing Purchaser:
                        -------------------------------------------------------

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