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                                               (COPY MARKED TO SHOW CONFIDENTIAL
                                                   TREATMENT CURRENTLY REQUESTED
                                                         FOR EXTENSION OF ORDER)
                                      


                                  CONFIDENTIAL
                                    TREATMENT
                                    REQUESTED
             
                                                                    EXHIBIT 10.9

                             RESEARCH COLLABORATION

                                       AND

                               LICENSE AGREEMENT


         THIS AGREEMENT is effective as of the 31st day of May, 1991 by and
between Vical Incorporated, a corporation organized and existing under the laws
of the State of Delaware, and having its principal offices at 9373 Towne Centre
Drive, San Diego, California 92121 ("VICAL"), and Merck & Co., Inc., a
corporation organized and existing under the laws of the State of New Jersey,
and having its principal office at 126 East Lincoln Avenue, Rahway, New Jersey
07065 ("MERCK").

         WHEREAS, VICAL possesses technology pertaining to the intramuscular
delivery of vaccines for the prevention of human infectious disease that consist
of DNA and/or RNA gene sequences derived from the genome of the infectious agent
which would prevent clinical disease from an infection (the "Technology"); and

         WHEREAS, MERCK wishes to collaborate with VICAL to develop an INFLUENZA
VACCINE as defined herein and an AIDS VACCINE as defined herein utilizing the
Technology and to obtain licenses under VICAL PATENT RIGHTS and VICAL KNOW-HOW.

         NOW, THEREFORE, VICAL and MERCK agree as follows:
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                            ARTICLE 1 - DEFINITIONS

1.1      AIDS VACCINE means a vaccine which would prevent an infection by Human
         Immunodeficiency Virus (HIV-1) and/or clinical diseases caused by
         infection with HIV-1 in humans.

1.2      INFLUENZA VACCINE means a vaccine which would prevent an infection
         and/or significant clinical diseases caused by any influenza virus in
         humans.

1.3      LICENSED PRODUCT means a bulk or finished AIDS VACCINE or INFLUENZA
         VACCINE, or other vaccine for the prevention of human infectious
         disease if licensed hereunder, which utilizes the Technology or
         technology which is developed by VICAL during and as a result of the
         RESEARCH COLLABORATION PROGRAM; provided, however, that if any such
         vaccine is also capable of being used for treatment of the same human
         infectious disease, then such therapeutic use of such vaccine shall
         also be considered a LICENSED PRODUCT for purposes of the license being
         granted by VICAL to MERCK under this Agreement.

1.4      COMBINATION PRODUCT means LICENSED PRODUCT in combination with one or
         more antigenically active components other than an AIDS VACCINE or an
         INFLUENZA VACCINE or other vaccine for the prevention of human
         infectious disease if licensed hereunder.

1.5      VICAL PATENT RIGHTS means (i) all patents and patent applications which
         relate to the Technology which are owned by VICAL as sole or joint
         owner or under which VICAL has licensable 
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         rights and which exist as of the effective date of this Agreement and
         any other similar future United States or foreign patent or patent
         applications which VICAL owns as sole or joint owner during and as a
         result of the RESEARCH COLLABORATION PROGRAM or under which VICAL
         acquires licensable rights during the RESEARCH COLLABORATION PROGRAM;
         and (ii) any divisions, continuations, or continuations-in-part of the
         patents and patent applications set forth above, including any reissue,
         reexamination or extension of the patent, any extended or restored
         term, and any confirmation patent, registration patent, or patent of
         addition.

1.6      VICAL KNOW-HOW means any information and data relevant to LICENSED
         PRODUCT not generally known including, but not limited to, procedures
         for experiments and tests and other protocols, results of
         experimentation and testing, fermentation and purification techniques
         and assay protocols, which are necessary or useful for MERCK in order
         to manufacture, use, develop, sell or seek approval to market LICENSED
         PRODUCT, in which VICAL has an ownership or licensable interest and
         which is presently in the possession of VICAL and/or which comes into
         the possession of VICAL during and as a result of the RESEARCH
         COLLABORATION PROGRAM.

1.7      MERCK KNOW-HOW means information and data not in MERCK's possession as
         of the effective date of this Agreement and developed solely under the
         RESEARCH COLLABORATION PROGRAM which in MERCK's reasonable discretion
         it determines are necessary and useful for VICAL in order to develop
         LICENSED PRODUCT.
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1.8      (a) NET SALES means the gross amount of sales of LICENSED PRODUCT
             invoiced to third parties less: 

             1.   the actual cost to MERCK of the devices for dispensing or
                  administering the LICENSED PRODUCT as well as diluents
                  or similar material which accompany the LICENSED PRODUCT
                  as it is sold;

             2.   trade and quantity discounts actually allowed;

             3.   returns, rebates and allowances;

             4.   retroactive price reductions; 

             5.   with regard to sales in the United States, [ * ]
                  of the amount invoiced to cover cash discounts, bad debt,
                  sales or excise taxes, transportation and insurance charges;
                  and with regard to sales outside the United States [ * ]
                  to include the above and additional special packaging, 
                  duties, and other governmental charges.

         (b) With respect to sales of COMBINATION PRODUCT, NET SALES shall be
             calculated on the basis of the sales price of the same dose of
             LICENSED PRODUCT being sold without other active ingredients if
             such exists, and if LICENSED PRODUCT is not sold without other
             active ingredients, NET SALES shall be calculated on the basis of
             the invoice price of the COMBINATION PRODUCT multiplied by a
             fraction, the numerator of which shall be the inventory cost of
             LICENSED PRODUCT in the COMBINATION PRODUCT and the denominator of
             which shall be the inventory cost (using the same method of
             accounting) of all of the active ingredients in the COMBINATION
             PRODUCT. Inventory cost shall be determined in 


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         accordance with MERCK's regular accounting methods. In no event shall
         the fraction be less than one over the number of active ingredients in
         the COMBINATION PRODUCT.

1.9      TERRITORY means all countries of the world.

1.10     AFFILIATE of VICAL or MERCK means any corporation or business entity
         which owns or controls VICAL or MERCK, or is under common control with
         VICAL or MERCK. The term "control" means possession, direct or
         indirect, of the power to direct or cause the direction of the
         management and policies, whether through the ownership of voting
         securities, by contract or otherwise.

1.11     RESEARCH COLLABORATION PROGRAM means the research related to the
         Technology to be conducted in accordance with this Agreement by VICAL
         and MERCK as set forth in Schedule B to develop LICENSED PRODUCT.

1.12     CONTRACT YEAR shall mean the one (1) year period beginning with the
         effective date of this Agreement and any subsequent one (1) year
         periods that this Agreement is in effect.

1.13     VALID PATENT RIGHTS shall mean issued and unexpired VICAL PATENT RIGHTS
         which have not been declared invalid by a court of competent
         jurisdiction from which no appeal can be or is taken.


              ARTICLE 2 - RESEARCH COLLABORATION AND DEVELOPMENT


2.1      Subject to MERCK's rights set forth in Article 8.2(b), the 
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         parties shall engage in a RESEARCH COLLABORATION PROGRAM for a three
         (3) year period beginning on the effective date of this Agreement in
         order to pursue the development of LICENSED PRODUCTS. The parties
         current understanding of the RESEARCH COLLABORATION PROGRAM is set
         forth in Schedule B attached hereto and made a part of this Agreement.

2.2      All activities of the parties involving the RESEARCH COLLABORATION
         PROGRAM shall be determined and approved by a majority of the members
         of a Review Committee, which shall consist of four (4) scientific
         representatives designated by MERCK and three (3) by VICAL. Each party
         shall make its initial designation of its representatives by notice to
         the other party within thirty (30) days after execution of this
         Agreement and shall cause its representatives to attend meetings of the
         Review Committee. The Review Committee shall meet no less frequently
         than once each calendar quarter.

2.3      No later than ninety (90) days prior to the expiration of each CONTRACT
         YEAR during the RESEARCH COLLABORATION PROGRAM, the parties shall meet
         to discuss in detail the progress of the RESEARCH COLLABORATION PROGRAM
         and agree upon its details, its short and long-term goals and
         prospective budget. In the event a significant development transpires
         during the twelve (12) month period which may affect the short- or
         long-term goals or budget of the RESEARCH COLLABORATION PROGRAM or
         methods of achieving said goals, the parties shall reconvene, reassess
         and, if mutually agreed, change such methods and goals and the RESEARCH
         COLLABORATION PROGRAM.
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2.4      Throughout the term of the RESEARCH COLLABORATION PROGRAM both parties
         shall share with each other know-how resulting from the RESEARCH
         COLLABORATION PROGRAM. MERCK and VICAL agree that such know-how shall
         be granted confidentiality by the receiving party and shall not be
         disclosed without the written prior consent of the supplying party,
         except as otherwise provided in this Agreement and to the extent that
         such information meets the exclusions set forth in Article 6.

2.5      Each party shall continue to own and retain proprietary rights to such
         know-how and patents licensed or transferred under this Agreement
         except as specifically provided herein. Inventions developed under the
         RESEARCH COLLABORATION PROGRAM shall be owned by the inventing party,
         which shall have the right to have patents filed to protect its rights.
         Joint inventions developed under the RESEARCH COLLABORATION PROGRAM
         shall be jointly owned, and the parties shall cooperate in the filing
         of patents relating to such joint inventions which MERCK shall
         prosecute and maintain and bear all expenses; provided, however, that
         VICAL shall be consulted by MERCK with respect to patent applications
         filed by MERCK with respect to joint inventions under this Article 2.5,
         and VICAL shall be given the opportunity to review all such patent
         applications prior to filing. [ * ] of all payments by MERCK under 
         this Article 2.5 as to such joint inventions shall be creditable 
         against royalties due VICAL under Article 8 of this Agreement; 
         however, no royalty payment when due, regardless of the number of 
         credits available to MERCK in accordance with the terms of this 
         Agreement, shall be reduced by more than [ * ].


[ * ]   CONFIDENTIAL TREATMENT REQUESTED
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2.6      Unless otherwise mutually agreed by MERCK and VICAL, responsibility for
         development of LICENSED PRODUCTS shall belong to MERCK. In this
         connection, MERCK shall use diligent efforts, consistent with those
         applied to other products of similar commercial value, to conduct a
         development program for each LICENSED PRODUCT to obtain regulatory
         approvals for such LICENSED PRODUCT as outlined in Article 13 below.
         Unless otherwise agreed, VICAL shall participate in such activities to
         the extent that MERCK deems such participation necessary in its sole
         discretion. Summary reports with respect to such development activities
         shall be made as outlined in Article 13 below.

                            
                           ARTICLE 3 - LICENSE GRANT


3.1      VICAL grants to MERCK an exclusive license under VICAL KNOW-HOW and
         VICAL PATENT RIGHTS to develop, make, have made, use and sell LICENSED
         PRODUCTS in the TERRITORY with the right to grant sublicenses to
         AFFILIATES of MERCK and those persons or entities through whom MERCK,
         in the normal course of its business collaborates in the manufacture
         and sale of its products; provided, however, that nothing in this
         Agreement shall prohibit VICAL from utilizing the VICAL KNOW-HOW and/or
         VICAL PATENT RIGHTS, exclusive of MERCK KNOW-HOW, to develop, make,
         have made, use and sell, either by itself or with one or more third
         parties, products for the treatment of human infectious diseases.

3.2      In accordance with the letter from MERCK to the Wisconsin Alumni
         Research Foundation ("WARF") dated May 29, 1991, a copy of which is
         attached hereto as Schedule D, the exclusive licenses 
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         granted under Article 3.1 hereof with respect to certain of the VICAL
         PATENT RIGHTS shall extend beyond and survive the termination of the
         License Agreement ("WARF License Agreement") dated and effective as of
         January 1, 1991 between VICAL and WARF in the event such termination
         occurs prior to the termination of this Agreement.

3.3      During the term of the RESEARCH COLLABORATION PROGRAM, all VICAL
         activities relating to the Technology and the development of a LICENSED
         PRODUCT for prevention of human infectious disease shall be conducted
         by VICAL exclusively for MERCK under the terms of this Agreement.

3.4      VICAL agrees that so long as the RESEARCH COLLABORATION PROGRAM
         continues in effect, at least thirty (30) days prior to entering into
         substantive business discussions with a third party with respect to a
         collaboration agreement in the field of vaccines for the prevention of
         auto-immune disease VICAL will provide MERCK representatives of the
         Review Committee, with a copy to the Executive Director, Corporate
         Licensing, with a one-time notice in writing of these proposed
         activities.


                  ARTICLE 4 - PATENT PROTECTION AND VALIDITY


4.1      VICAL represents and warrants to MERCK that as of the effective date of
         this Agreement it (i) has the right to grant the licenses granted under
         Article 3.1 hereof and to provide the VICAL KNOW-HOW to MERCK pursuant
         to this Agreement, (ii) to the extent lawfully allowed by the Wisconsin
         Alumni Research Foundation will maintain in 
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         full force and effect the WARF License Agreement for so long as this
         Agreement remains in force, and (iii) will take no action with respect
         to the WARF LICENSE AGREEMENT which would conflict with MERCK's rights
         under this Agreement. MERCK may elect to terminate this Agreement in
         the event VICAL is determined by WARF to be in breach of the WARF
         LICENSE AGREEMENT. MERCK acknowledges that (i) VICAL has entered into
         the WARF License Agreement, (ii) MERCK has reviewed such Agreement, in
         the form which is attached as Schedule C, (iii) that any sublicense
         from VICAL to MERCK with respect thereto shall be subject, to the
         extent set forth in Schedule D, to the terms of the WARF License
         Agreement, including, without limitation, Section 2C of such Agreement
         with respect to sublicenses which extend beyond its termination.

4.2      MERCK represents and warrants that it has the full right to enter into
         this Agreement and that it neither has made nor will make any
         commitments to others in conflict with or in derogation of this
         Agreement.

4.3      Nothing in this Agreement shall be construed as a warranty or
         representation by VICAL as to the validity or scope of any VICAL PATENT
         RIGHTS. Nothing in this Agreement shall be construed as a warranty or
         representation that any LICENSED PRODUCT made, used or sold, or
         otherwise disposed of under any license granted by this Agreement, or
         any use of the VICAL KNOW-HOW, is or will be free from infringement of
         patents of third 
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         parties. VICAL makes no warranty or representation as to the 
         viability, safety or efficacy of any LICENSED PRODUCT. THE WARRANTIES 
         EXPRESSLY SET FORTH HEREIN ARE EXCLUSIVE AND NO OTHER WARRANTY, 
         WRITTEN OR ORAL, INCLUDING WARRANTIES OF MERCHANTABILITY OR FITNESS 
         FOR A PARTICULAR PURPOSE, IS EXPRESSED OR IMPLIED.

4.4      To the extent it may lawfully do so, and except to the extent MERCK has
         agreed to do so with respect to joint inventions as set forth in
         Article 2.5 above VICAL agrees to file, prosecute and maintain in such
         jurisdictions as it reasonably deems commercially appropriate after
         consultation with MERCK the VICAL PATENT RIGHTS owned in whole or in
         part by VICAL and licensed to MERCK under this Agreement, including
         prosecution of an interference. VICAL shall give notice to MERCK of any
         decision to cease prosecution and maintenance of such VICAL PATENT
         RIGHTS and, in such case, shall permit MERCK at its sole discretion to
         continue prosecution or maintenance at its own expense. If MERCK elects
         to continue prosecution or maintenance, VICAL shall execute such
         documents and perform such acts at VICAL's expense as may be reasonably
         necessary for MERCK to continue prosecution or maintenance. [ * ] of 
         all expenses and costs incurred by MERCK to continue prosecution and 
         subsequent maintenance of VICAL PATENT RIGHTS shall be fully 
         creditable against royalties due under Article 8 of this Agreement; 
         however, no royalty payment when due, regardless of the number of such 
         credits available to MERCK in accordance with the 


[ * ]   CONFIDENTIAL TREATMENT REQUESTED
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         terms of this Agreement, shall be reduced by more than [ * ].

4.5      VICAL shall protect, where it reasonably determines that it is
         commercially advisable to do so after consultation with MERCK the VICAL
         PATENT RIGHTS licensed to MERCK under this Agreement against any third
         party who infringes the VICAL PATENT RIGHTS by development,
         manufacture, use or sale of LICENSED PRODUCT for which MERCK retains a
         license under this Agreement.

4.6      In the event VICAL institutes an action at its expense against third
         party infringers with respect to LICENSED PRODUCT or takes appropriate
         action to defend the VICAL PATENT RIGHTS, MERCK hereby agrees to
         cooperate fully with VICAL and any recovery obtained by VICAL as a
         result of such proceeding or other actions, whether obtained by
         settlement or otherwise, shall be retained by VICAL, except that VICAL
         shall pay to MERCK any reasonable expenses incurred in assisting in
         such action (excluding counsel fees) and any amount of the recovery
         attributable to lost profit on lost sales by MERCK after deduction of
         the royalty due VICAL on such sales under Article 8.

4.7      If within sixty (60) days of becoming aware of the infringement of the
         VICAL PATENT RIGHTS for which MERCK retains a license under this
         Agreement or unauthorized use of the VICAL KNOW-HOW with respect to
         such a LICENSED PRODUCT, VICAL decides not to institute an 



[ * ]   CONFIDENTIAL TREATMENT REQUESTED
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         infringement suit or take other reasonable action to protect the VICAL
         PATENT RIGHTS and VICAL KNOW-HOW, MERCK shall have the right to
         institute such suit or take other appropriate action at its own expense
         in the name of VICAL or MERCK or both. In such event, VICAL shall
         cooperate fully with MERCK in its efforts to protect the VICAL PATENT
         RIGHTS and the VICAL KNOW-HOW. Any recovery obtained by MERCK as a
         result of such proceeding, by settlement or otherwise, shall be the
         property of MERCK, provided, however, that MERCK shall pay to VICAL any
         reasonable expenses incurred by VICAL in assisting with such action
         (other than counsel fees) and the applicable royalty VICAL would have
         received on sales lost by MERCK to the extent that the recovery
         includes sales lost by MERCK.

4.8      VICAL and MERCK each shall immediately give notice to the other of any
         certification of which they become aware filed under the U.S. "Drug
         Price Competition and Patent Term Restoration Act of 1984" claiming
         that a patent covering LICENSED PRODUCT is invalid or that infringement
         will not arise from the manufacture, use or sale of LICENSED PRODUCT by
         a third party. If VICAL or MERCK (depending on which party is defending
         the patent) decides not to bring infringement proceedings against the
         entity making such a certification, such party shall give notice to the
         other party of its decision not to bring suit within twenty-one (21)
         days after receipt of notice of such certification. The party receiving
         such notice may then, but is not required to, bring suit 
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         against the party that filed the certification. Any suit by MERCK or
         VICAL shall either be in the name of MERCK or in the name of VICAL, or
         jointly by MERCK and VICAL, as may be required by law. For this
         purpose, the party not bringing suit shall execute such legal papers
         necessary for the prosecution of such suit as may be reasonably
         requested by the party bringing suit.

4.9      VICAL shall promptly give notice to MERCK of the grant, lapse,
         revocation, surrender, invalidation or abandonment of any VICAL PATENT
         RIGHTS licensed to MERCK for which VICAL is responsible for the
         application, prosecution and maintenance.

4.10     The parties hereto shall cooperate with each other in gaining patent
         term extension where applicable to VICAL PATENT RIGHTS.

4.11     In any country where a compulsory license must be granted involving
         VICAL PATENT RIGHTS, the exclusive rights and license granted to MERCK
         shall not prohibit or prevent the granting of a compulsory license
         under the VICAL PATENT RIGHTS and any royalty payable by MERCK shall
         not be greater than the royalty payable by the compulsory licensee.
         VICAL shall consult with MERCK with respect to any circumstances in
         which it may be required to grant a compulsory license.
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                      ARTICLE 5 - TRANSFER OF INFORMATION


5.1      Each party shall disclose upon request by the other party all VICAL or
         MERCK KNOW-HOW, as applicable, not already disclosed. During the term
         of the RESEARCH COLLABORATION PROGRAM, VICAL shall also disclose to
         MERCK on an ongoing basis all VICAL KNOW-HOW developed in connection
         with the RESEARCH COLLABORATION PROGRAM.

5.2      VICAL shall have the option to acquire a nonexclusive, worldwide
         license to any technology developed solely by MERCK under the RESEARCH
         COLLABORATION PROGRAM and MERCK's interest in any technology developed
         jointly with VICAL under the RESEARCH COLLABORATION PROGRAM
         (hereinafter referred to as "MERCK Licensed Technology") which would be
         necessary for VICAL to develop vaccines which MERCK has not elected to
         develop or for which it has not pursued development as required under
         the terms of this Agreement.

5.3      Under the option granted in Article 5.2, VICAL shall have the right to
         grant sublicenses to subsidiaries without consent and to third parties
         approved in advance by MERCK (which approval shall not be unreasonably
         withheld) only in the event that VICAL elects not to develop and/or
         market the MERCK Licensed Technology. The right to grant sublicenses to
         subsidiaries without consent and approved third parties shall not
         include the right of such subsidiaries or approved third parties to
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         grant further sublicenses except as MERCK may otherwise agree in its
         sole discretion.

5.4      In the event VICAL elects to exercise the option granted in Article
         5.2, the parties shall negotiate in good faith a License Agreement
         which shall provide for the payment of royalties reasonable under the
         circumstances by VICAL to MERCK on net sales by VICAL, its Affiliates
         or sublicensee of such vaccine products utilizing the MERCK Licensed
         Technology and which shall provide for such other provisions
         customarily included in such an Agreement.


                          ARTICLE 6 - CONFIDENTIALITY


6.1      Except as otherwise provided in this Agreement, all VICAL or MERCK
         KNOW-HOW or other confidential information which is received by a party
         during the term of this Agreement shall be maintained in confidence by
         the recipient and shall not be disclosed to any other person, firm, or
         agency, governmental or private, without the prior written consent of
         the other party, except to the extent that such materials or
         information:

6.1.1    is known by recipient at the time of its receipt as documented in
         written records, or

6.1.2    is properly in the public domain, or
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6.1.3    is subsequently disclosed to a party by a third party not under an
         obligation of confidentiality to the disclosing party, or

6.1.4    is developed independently of VICAL or MERCK KNOW-HOW, as the case may
         be, or information received by recipient as documented in written
         records, or

6.1.5    is required to be disclosed to governmental agencies in order to gain
         approval to sell LICENSED PRODUCT (but only to the extent that such
         disclosure is required), or

6.1.6    is necessary to be disclosed to sublicensees, agents, consultants,
         AFFILIATES and/or other third parties for the research and development
         and/or sale and marketing of LICENSED PRODUCT under this Agreement,
         which entities first agree to be bound by the confidentiality
         obligations contained in this Agreement. It is understood that each
         party may disclose confidential information with the prior consent and
         approval of the other party.


                   ARTICLE 7 - ADVERSE EXPERIENCE REPORTING


7.1      The parties agree throughout the duration of this Agreement to notify
         each other immediately of any information concerning any serious or
         unexpected side effect, injury, toxicity or sensitivity reaction, or
         any unexpected incidence, and the severity thereof, associated with the
         clinical uses, studies, investigations, tests and marketing of the
         LICENSED 
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         PRODUCT. "Serious", as used in this paragraph, refers to an experience
         which results in death, permanent or substantial disability, in-patient
         hospitalization, prolongation of hospitalization; or is a congenital
         anomaly, cancer, the result of an overdose, or life threatening.
         "Unexpected", as used in this paragraph for a non-marketed LICENSED
         PRODUCT, is one that is not identified in nature, severity, or
         frequency in the current clinical investigator's confidential
         information brochure. "Unexpected", as used in this paragraph for a
         marketed product, is one that is not listed in the current labeling for
         the LICENSED PRODUCT and includes an event that may be symptomatically
         and pathophysiologically related to an event listed in the labeling but
         differs from the event because of increased frequency or greater
         severity or specificity.


                 ARTICLE 8 - PAYMENTS AND OTHER CONSIDERATIONS


8.1      MERCK shall make a nonrefundable payment in consideration of VICAL's
         research of [ * ] upon execution of this Agreement.

8.2      (a) In support of mutually agreed upon research to be conducted by
         VICAL for the first CONTRACT YEAR of the RESEARCH COLLABORATION
         PROGRAM, (which is described in Schedule B attached hereto) MERCK shall
         pay VICAL [ * ] payable upon the execution date 




[ * ]   CONFIDENTIAL TREATMENT REQUESTED
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         of this Agreement. It is understood that this amount will support five
         and one-half (5.5) research scientists.

(b)      In the event MERCK has not terminated the Agreement at the end of the
         first CONTRACT YEAR as provided in Article 12.3 MERCK shall fund the
         PROGRAM for the second and, if the Agreement continues to remain in
         effect, the third CONTRACT YEAR, at no less than four hundred
         ninety-five thousand dollars ($495,000) for each such year or such
         other greater dollar amount as the Review Committee unanimously and
         reasonably determines shall be necessary to fund VICAL's activities
         contemplated for such additional CONTRACT YEAR based upon the RESEARCH
         COLLABORATION PROGRAM set forth in the attached Schedule B. It is
         understood that this amount will support a minimum of two and three
         quarter (2.75) research scientists.

(c)      MERCK's rights and obligations under Article 3.1 of this Agreement
         shall terminate in the event MERCK elects not to fund the PROGRAM for
         the second and third CONTRACT YEAR as set forth herein unless MERCK
         pays VICAL, to reflect VICAL's continuing research the nonrefundable
         sum of five hundred thousand dollars ($500,000) for AIDS VACCINE and/or
         five hundred thousand dollars ($500,000) for INFLUENZA VACCINE (an
         aggregate of one million dollars ($1,000,000.) for both) within thirty
         (30) days prior to the end of the second CONTRACT 
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         YEAR. Prior to such time, and afterwards in the event such payments are
         made, MERCK's obligations with respect to AIDS VACCINE and INFLUENZA
         VACCINE shall continue as set forth under the terms and conditions of
         this Agreement.

8.3     (a)    As further consideration and in recognition of the research to be
               performed by VICAL under this Agreement and subject to the
               provisions of Article 12, MERCK shall make the nonrefundable
               research milestone payments provided for in Schedule A for each
               of the first five (5) vaccines for the prevention of human
               infectious disease selected by MERCK in accordance with the terms
               of this Agreement including AIDS VACCINE and INFLUENZA VACCINE.
               Such payments shall be made within thirty (30) days of the
               achievement of the milestone.

        (b)    As an additional research milestone payment, in the event that a
               United States Patent covering LICENSED PRODUCT issues to VICAL
               while this Agreement is in effect and prior to LICENSED PRODUCT
               entering Phase III clinical studies, then MERCK shall pay to
               VICAL, on a one time basis, the nonrefundable sum of [ * ]
               at the end of the first CONTRACT YEAR or upon completion of 
               VICAL's duties under Proof of Principle experimental studies as 
               described in Schedule B attached hereto whichever occurs later 
               or as soon thereafter as said patent issues and the  
               nonrefundable sum of [ * ]



[ * ]   CONFIDENTIAL TREATMENT REQUESTED
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               [ * ] on initiation of Phase III clinical studies. If such
               patent issues after initiation of Phase III clinical studies and
               while this Agreement remains in effect, then MERCK shall pay to
               VICAL, on a one time basis, the nonrefundable sum of [ * ] at 
               the time such patent issues.

        (c)    MERCK shall have the right to an option to extend the rights
               granted in Article 3.1 hereof through the second CONTRACT YEAR to
               vaccines used for prevention of all other human infectious
               diseases, to a RAS oncogene vaccine derived from the Kirsten -,
               Harvey -, or N-RAS oncogene for the prevention of ras oncogene
               associated tumors, and to a human papilloma virus vaccine derived
               from the genome of the human papilloma virus for the prevention
               of papilloma associated tumors and lesions other than LICENSED
               PRODUCT and shall have such right to do so (i) in the first
               CONTRACT YEAR at no additional cost and (ii) in the second
               CONTRACT YEAR by paying VICAL the sum of one million dollars
               ($1,000,000) at the end of the first CONTRACT YEAR or at the
               successful completion of VICAL's Proof of Principle experimental
               studies (as described in Schedule B attached), whichever is
               later. Thereafter, following only the second and third CONTRACT
               YEARS, respectively, MERCK shall again have the right to an
               option to extend the rights granted in Article 3.1 hereof on a
               year-by-year and vaccine-by-vaccine basis by paying to VICAL the
               amount of S250,000 per 



[ * ]   CONFIDENTIAL TREATMENT REQUESTED
   22
                                     - 22 -


               [ * ]; provided, however, that MERCK shall use its good faith 
               efforts to begin or continue, as the case may be, conduct of a 
               development program as outlined in this Agreement with respect to
               any vaccines for which it has made the above described payments,
               if so required. If MERCK determines to so extend the rights as
               described above, payments for such extension shall be made no
               later than thirty (30) days preceding the end of the second or
               third CONTRACT YEAR, as the case may be. MERCK shall lose all
               right to any vaccine for which it does not make the above
               described payments, if so required, or to any vaccines for which
               MERCK already has the rights but the development of which it has
               not been pursued as required by the terms of this Agreement;
               provided, however, during the term of this Agreement that at
               least thirty (30) days prior to entering into substantive
               business discussions with a third party with respect to a
               collaborative agreement regarding a vaccine for which MERCK has
               not made the above described payments, if so required, or pursued
               development as required, VICAL will provide MERCK representatives
               of the Review Committee with a one-time notice in writing of
               these proposed activities. A copy of such notice shall also be
               provided to the Executive Director, Corporate Licensing. 



[ * ]   CONFIDENTIAL TREATMENT REQUESTED
   23
                                     - 23 -



         (d)   If MERCK determines to exercise the options for which it has paid
               VICAL the sums as required in Article 8.3(c) above it shall do so
               on or before the end of the second, third or fourth CONTRACT
               YEARS, as the case may be. In such event MERCK shall pay VICAL, a
               license fee of [ * ] of the payment made under Article 8.3(c) 
               (ii) shall be creditable against the license fee paid hereunder.

         (e)   The terms and conditions of this Agreement will be applicable to
               licenses of additional infectious disease vaccines provided that
               the only consideration under such agreements shall be the
               royalties and milestones provided herein.

8.4      In consideration of the license granted in Article 3, royalties shall
         be payable to VICAL in each calendar year in the amount of [ * ] of 
         NET SALES by MERCK, its AFFILIATES or permitted sublicensees of each 
         LICENSED PRODUCT which is covered by VALID PATENT RIGHTS.

8.5      In the event a LICENSED PRODUCT sold in a country is not covered by
         VALID PATENT RIGHTS (including countries where patents have been
         applied for but have not yet issued), then MERCK shall pay know-how
         royalties of [ * ] on NET SALES of such LICENSED PRODUCT in such 
         country for 



[ * ]   CONFIDENTIAL TREATMENT REQUESTED
   24
                                     - 24 -



         a period of [ * ] from the date of first commercial sale of LICENSED 
         PRODUCT in such country. The royalties payable under this section 
         shall be in lieu of patent royalties and not additive where patents 
         later issue in a country.

8.6      Sales between MERCK and its sublicensees or AFFILIATES, or among such
         AFFILIATES and sublicensees, shall not be subject to royalty, but in
         such cases royalty shall be calculated upon MERCK's or its
         sublicensee's or AFFILIATES's NET SALES to an independent third party.

8.7      The obligation to pay royalties is imposed only once with respect to
         the same unit of LICENSED PRODUCT.

8.8      If MERCK is required to pay cumulative royalties in excess of [ * ]
         of Net Sales for additional licenses required to commercialize a 
         particular LICENSED PRODUCT, the royalties payable to VICAL herewith 
         with respect to such LICENSED PRODUCT shall be reduced by [ * ] of the 
         additional royalties beyond such [ * ] figure; provided however that 
         in no event shall the royalties due VICAL hereunder, after taking 
         into account the above reduction be reduced below [ * ].

8.9      In the event that VICAL seeks to obtain additional licenses from third
         parties which may enhance or improve one or more LICENSED PRODUCTS,
         VICAL shall 



[ * ]   CONFIDENTIAL TREATMENT REQUESTED
   25
                                     - 25 -




         discuss with MERCK its interest in such additional licenses and on what
         terms such licenses may be obtained by VICAL or MERCK.


                      ARTICLE 9 - ACCOUNTING AND REPORTS


9.1      MERCK shall deliver to VICAL within sixty (60) days after the end of
         each calendar quarter a written account, including quantities and
         monetary amounts of MERCK's and MERCK's AFFILIATE's and sublicensee's
         sales subject to royalty payments and the amount of the royalty payment
         due to VICAL for such quarter. In the case of sales outside the United
         States, "calendar quarter" shall mean the respective periods of three
         (3) consecutive calendar months ending on February 28, May 31, August
         31, and November 30. With respect to these "calendar quarters",
         royalties shall be calculated for such periods but accounted for and
         paid within sixty (60) days after the end of each regular calendar
         quarter as set forth above. Any such payments by MERCK that are not
         paid on or before the date such payments are due under this Agreement
         shall bear interest to the extent permitted by applicable law at the
         reference rate of interest as reported by Bank of America NT & SA in
         San Francisco, California from time to time, calculated on the number
         of days such payment is delinquent.

9.2      When MERCK delivers the accounting to VICAL, MERCK shall also deliver
         all royalty payments due to VICAL for the calendar quarter.
   26
                                     - 26 -


9.3      MERCK shall keep accurate records in sufficient detail to enable the
         amounts due to VICAL to be determined. Upon VICAL's request, MERCK
         shall permit an independent, certified public accountant selected by
         VICAL, except one to whom MERCK has reasonable objection, to have
         access during ordinary business hours to MERCK's records necessary to
         determine the correctness of any report or payment made in respect to
         any calendar quarter and obtain information as to the amount payable to
         VICAL for any such period in case of MERCK's failure to report or make
         payment. Such examination shall be at VICAL's expense and shall not
         take place more than once each year. These rights with respect to any
         year shall terminate two (2) years after the end of any such year;
         provided, however, that if such examination discloses that the
         royalties payable for the examination period are more than one hundred
         five percent (105%) of the royalties actually paid for such period,
         MERCK shall pay the expenses of the accountant. Information supplied to
         VICAL by such independent, certified public accountants shall not
         include any proprietary information not required to be disclosed under
         other sections of this Agreement.

9.4      All payments to be made by MERCK to VICAL under this Agreement shall be
         made in United States dollars by bank wire transfer in immediately
         available funds. In the case of sales outside the United States, the
         rate of exchange to be used in computing the amount of currency
         equivalent in United States dollars due VICAL 
   27
                                     - 27 -



         shall be made at the rate of exchange, utilized by MERCK in its
         worldwide accounting system, prevailing on the fourth-to-the last
         business day of the calendar quarter.


                           ARTICLE 10 - MANUFACTURE


10.1     In the event that MERCK decides to develop any LICENSED PRODUCT under
         the terms of this Agreement and subject to the terms and conditions of
         a customary manufacturing and supply agreement, MERCK shall discuss
         with and, in its sole discretion, may grant to VICAL the right to
         manufacture a part of MERCK's requirements of LICENSED PRODUCT.


                             ARTICLE 11 - DURATION


11.1     This Agreement becomes effective as of the day and year first above
         written and may be terminated as set forth in Article 12 hereof and
         otherwise remains in effect until MERCK is no longer obligated to make
         payments to VICAL pursuant to this Agreement. At such time, the rights
         and licenses granted to MERCK under this Agreement shall be fully paid
         up and shall continue in full force and effect.


                           ARTICLE 12 - TERMINATION


12.1     Upon any material breach by either party under this Agreement, in
         addition to any other remedy it may 
   28
                                     - 28 -


         have, the other party may terminate this Agreement by ninety (90) days
         written notice to the breaching party, specifying the material breach,
         default or other defect. The termination becomes effective at the end
         of the ninety (90) day period unless the breaching party cures the
         breach during the ninety (90) day period.

12.2     Either party may terminate this Agreement without notice if the other
         party becomes insolvent, makes an assignment for the benefit of
         creditors, is the subject of proceedings in voluntary or involuntary
         bankruptcy instituted on behalf of or against such party, or has a
         receiver or trustee appointed for substantially all of its property;
         provided that in the case of an involuntary bankruptcy proceeding such
         right to terminate shall only become effective if the party consents
         thereto or such proceeding is not dismissed within ninety (90) days
         after the filing thereof.

12.3     Notwithstanding any other provisions of this Agreement to the contrary,
         this Agreement may be terminated by MERCK at the end of the first
         CONTRACT YEAR or thereafter upon ninety (90) days prior written notice
         given to VICAL. In the event of such termination, no further sums shall
         be payable by MERCK under this Agreement and the licenses and rights
         granted to MERCK shall be terminated except as set forth in Article
         8.2(c) respecting rights and obligations MERCK has 
   29
                                     - 29 -


         elected to continue for AIDS VACCINE and human INFLUENZA VACCINE. Upon
         such termination, MERCK shall (i) return to VICAL any and all VICAL
         KNOW-HOW or other VICAL technology related to VICAL PATENT RIGHTS
         provided that MERCK may retain, in its Legal files, a copy of such
         written KNOW-HOW or technology for record purposes and (ii) disclose to
         VICAL and license to VICAL in accordance with Article 5.2 any
         previously undisclosed MERCK KNOW-HOW or other technology developed by
         MERCK under the RESEARCH COLLABORATION PROGRAM.

12.4     Any expiration or early termination of this Agreement shall be without
         prejudice to the rights of either party against the other accrued or
         accruing under this Agreement prior to termination, including the
         obligation to pay royalties for LICENSED PRODUCT sold prior to such
         termination.

12.5     Upon termination of this Agreement, all provisions regarding
         confidentiality shall continue in full force and effect until seven (7)
         years after termination. Additionally, upon such termination, each
         party shall continue to be a joint owner of any joint inventions
         developed under the RESEARCH COLLABORATION PROGRAM.


                             ARTICLE 13 - EFFORTS


13.1     MERCK shall use diligent efforts, consistent with those applied to
         other products of similar commercial value, to design and complete all
         (except as otherwise provided) toxicological, pharmacological and
         clinical 
   30
                                     - 30 -


         investigations, to the extent reasonably deemed necessary by MERCK, to
         obtain and maintain government approvals to import, register and market
         any LICENSED PRODUCT in each country in the Territory, in which MERCK
         determines that it is commercially reasonable to enter and to market
         such LICENSED PRODUCTS in such countries. All such regulatory
         applications for approval to market any LICENSED PRODUCTS in the
         Territory shall be filed in the name of and owned by MERCK. MERCK shall
         notify VICAL each time it submits an application for government
         registration and marketing approval for any LICENSED PRODUCT, shall
         notify VICAL of any action taken on any application, and shall promptly
         advise VICAL when any government approval to market any LICENSED
         PRODUCT has been obtained in the Territory.

13.2     MERCK shall furnish VICAL with semiannual, summary progress reports
         with respect to MERCK's efforts under Articles 2.6 and 13.1 above as to
         each LICENSED PRODUCT until the date of first commercial sale of such
         PRODUCT. Prior to such first commercial sale and so long as this
         Agreement remains in effect, if VICAL, in order for VICAL to develop
         vaccines which MERCK has elected not to develop under this Agreement,
         requests access to information, data or technology in
         MERCK's possession, relating exclusively to the LICENSED PRODUCTS, 
         MERCK may consider such request, and in its sole discretion, 
         determine if and on what terms access to such information, data or 
         technology may be
   31
                                     - 31 -



         obtained by VICAL


                          ARTICLE 14 - GOVERNING LAW


14.1     This Agreement shall be construed and the respective rights of the
         parties hereto determined according to the substantive laws of the
         State of New Jersey notwithstanding the provisions governing conflict
         of laws under such law to the contrary.


                            ARTICLE 15 - ASSIGNMENT


15.1     Neither party may assign this Agreement in whole or in part without the
         consent of the other, except that no such consent shall be required (i)
         in the case of MERCK if such assignment occurs in connection with the
         sale of all or substantially all of the business and assets of MERCK
         related to vaccines, or if MERCK assigns this Agreement to an AFFILIATE
         and (ii) in the case of VICAL, if such assignment occurs in connection
         with the acquisition, merger, or sale of all or substantially all of
         the assets of VICAL related to the Technology or a change in control of
         VICAL. Assignment of this Agreement shall not release the assigning
         party of its performance obligations under this agreement without the
         consent of the other party which consent shall not be unreasonably
         withheld.

                            ARTICLE 16 - SEVERANCE

16.1     If any provision of this Agreement is held to be invalid or
         unenforceable under the laws of either 
   32
                                     - 32 -



         jurisdiction of the parties, all other provisions shall nevertheless
         continue in full force and effect, unless there is a material change in
         the benefits and/or rights received under this Agreement.

                            ARTICLE 17 - AMENDMENT


17.1     This Agreement constitutes the entire agreement between the parties and
         supersedes all previous arrangements whether written or oral. Any
         amendment or modification to this Agreement shall be made in writing
         signed by both parties.


                              ARTICLE 18 - NOTICE

18.1     Notices to VICAL shall be addressed to:

            Vical Incorporated
            9373 Towne Centre Drive
            San Diego, California 92121
            Attention: President

            Copy to:

            Thomas E. Sparks, Jr.
            Pillsbury, Madison & Sutro
            235 Montgomery St.
            San Francisco, CA 94104


Notices to MERCK shall be addressed to:

            Merck & Co., Inc.
            P.O. Box 2000
            Rahway, New Jersey  07065

            Attention:  Office of the Secretary

         Either party may change its address by giving notice to the other party
         in the manner herein provided. Any notice required or provided for by
         the terms of this Agreement shall be in writing and sent by registered
         or certified
   33
                                     - 33 -


         mail, return receipt requested, postage prepaid or by express courier
         services providing evidence of delivery and properly addressed in
         accordance with the paragraph above. The effective date of notice shall
         be the actual date of receipt by VICAL or MERCK.


                          ARTICLE 19 - FORCE MAJEURE

19.1     No failure or omission by the parties hereto in the performance of any
         obligation of this Agreement shall be deemed a breach of this Agreement
         or create any liability if the same shall arise from any cause or
         causes beyond the control of the parties, including, but not limited
         to, the following: act of God; acts or omissions of any government; any
         rules, regulations or orders issued by any governmental authority or by
         any officer department, agency or instrumentality thereof; fire; storm;
         flood; earthquake; accident; war; rebellion; insurrection; riot;
         invasion; strikes; and lockouts and provided that such failure or
         omission resulting from one of the above causes is cured as soon as is
         practicable after the occurrence of one or more of the above-mentioned
         causes.


                ARTICLE 20 - PUBLIC ANNOUNCEMENTS/MISCELLANEOUS


20.1     Any public announcements or similar publicity with respect to this
         Agreement or the transactions contemplated herein shall be at such time
         and in such manner as VICAL and MERCK shall agree, provided that
         nothing herein shall prevent either party upon notice to the other from
         making such 
   34
                                     - 34 -


         public announcements as such party's legal obligations require. VICAL
         may disclose this Agreement or the transactions contemplated thereby to
         investors or potential investors in VICAL or to financial institutions
         for the purpose of obtaining financing in the event that such
         disclosure is reasonably required to obtain an investment or financing
         and MERCK will have the opportunity to review such disclosure in
         advance.

20.2     During the term of this Agreement, each party shall be free to submit
         scientific manuscripts for publication regarding the work of the
         RESEARCH COLLABORATION PROGRAM, provided that the party wishing to
         publish shall submit to the other party all underlying data and the
         final draft of all such publications, whether they are to be presented
         orally or in written form, at least thirty (30) days prior to
         presentation or submission for publication. As may be applicable, the
         reviewing party shall advise the publishing party as to the
         patentability of any inventions to be disclosed and both parties shall
         work together to protect any and all proprietary rights.

                          ARTICLE 21 - INDEMNIFICATION


21.1     Each party shall indemnify and hold the other party harmless, and
         hereby forever releases and discharges the other party, from and
         against all claims, demands, liabilities, damages and expenses
         (including attorneys' fees) arising out of negligence of the
         indemnifying party, its AFFILIATES or permitted sublicensees in
         connection with 
   35
                                     - 35 -




         the work performed during the RESEARCH COLLABORATION PROGRAM.

21.2     MERCK agrees to indemnify and hold VICAL, its employees and agents
         harmless from and against any claims, demands, liabilities, damages and
         expenses (including attorneys' fees) arising out of MERCK's
         development, testing, use, manufacture, marketing or other activities
         contemplated under this Agreement with respect to LICENSED PRODUCTS.

21.3     A party (the "Indemnitee") that intends to claim indemnification under
         this Article 21 shall promptly notify the other party (the
         "Indemnitor") in writing of any loss, claim, damage, liability or
         action in respect of which the Indemnitee or any of its AFFILIATES
         intend to claim such indemnification, and the Indemnitor shall have the
         right to participate in, and, to the extent the Indemnitor so desires,
         jointly with any other Indemnitor similarly noticed, to assume the
         defense thereof with counsel mutually satisfactory to the parties;
         provided, however, that an Indemnitee shall have the right to retain
         its own counsel, with the fees and expenses to be paid by the
         Indemnitee, if representation of such Indemnitee by the counsel
         retained by the Indemnitor would be inappropriate due to actual or
         potential differing interests between such Indemnitee and any other
         party represented by such counsel in such proceedings. The indemnity
         agreement in this Article 21 shall not apply to amounts paid in
         settlement of any loss, claim, damage, liability or action if such
         settlement is effected without the consent of the 
   36
                                     - 36 -


         Indemnitor, which consent shall not be withheld unreasonably. The
         failure to deliver written notice to the Indemnitor within a reasonable
         time after the commencement of any such action, if prejudicial to its
         ability to defend such action, shall relieve such Indemnitor of any
         liability to the Indemnitees under this Article 21, but the omission so
         to deliver written notice to the Indemnitor will not relieve it of any
         liability that it may have to any Indemnitee otherwise than under this
         Article 21. The Indemnitee under this Article 21, its employees and
         agents, shall cooperate fully with the Indemnitor and its legal
         representatives in the investigation of any action, claim or liability
         covered by this indemnification.

21.4     MERCK shall maintain product liability insurance with respect to
         manufacture and sales of LICENSED PRODUCTS by MERCK, its AFFILIATES or
         permitted sublicensees in such form and in such amount as MERCK
         customarily maintains with respect to sales of its other vaccine
         products, and VICAL shall be named as an additional insured on any such
         product liability insurance policy. MERCK shall maintain such insurance
         for so long as it continues to manufacture or sell any LICENSED
         PRODUCTS, and thereafter for so long as MERCK maintains insurance for
         itself covering such manufacture or sales.


                          ARTICLE 22 - MISCELLANEOUS


22.1     This Agreement, including all Schedules attached hereto, 
   37
                                     - 37 -


         contains the entire understanding of the parties with respect to the
         subject matter hereof. All express or implied agreements and
         understandings, either oral or written, heretofore made are expressly
         merged in and made a part of this Agreement. This Agreement may be
         amended, or any term hereof modified, only by a written instrument duly
         executed by both parties hereto.

22.2     It is expressly agreed that MERCK and VICAL shall be independent
         contractors and that the relationship between the two parties shall not
         constitute a partnership, joint venture or agency. Neither MERCK nor
         VICAL shall have the authority to make any statements, representations
         or commitments of any kind, or to take any action, which shall be
         binding on the other party without the prior written consent of such
         other party.

22.3     This Agreement may be executed in two or more counterparts, each of
         which shall be deemed an original, but all of which together shall
         constitute one and the same instrument.





MERCK & CO., INC.                                VICAL INCORPORATED


By  /s/ Edward M. Scolnick                       By  /s/ Dannie H. King
   --------------------------------                 ----------------------------
   Edward M. Scolnick, M.D.


Title:   President, MSDRL                        Title:  President and CEO


Date:  6/6/91                                    Date:  5/31/91
   38
                                     - 38 -


                                   SCHEDULE A

                                   MILESTONES



         FIRST ENTRY INTO PHASE I (or its
         equivalent) IN A MAJOR MARKET COUNTRY                          $1.0M


         FIRST ENTRY INTO PHASE III (or its
         equivalent) IN A MAJOR MARKET COUNTRY                          [ * ]


         FIRST FILING OF PRODUCT LICENSE
         APPLICATION IN A MAJOR MARKET COUNTRY                          [ * ]


         FIRST APPROVAL OF PRODUCT LICENSE
         APPLICATION IN A MAJOR MARKET COUNTRY                          [ * ]


         "Major Market Country" shall mean the United States, EEC
         countries, Canada, or Japan.




[ * ]   CONFIDENTIAL TREATMENT REQUESTED
   39
                                   SCHEDULE B

                         RESEARCH COLLABORATION PROGRAM

                                     YEAR 1





                                     [ * ]





[ * ]   CONFIDENTIAL TREATMENT REQUESTED
   40
SCHEDULE B (continued)


                                     [ * ]




[ * ]   CONFIDENTIAL TREATMENT REQUESTED
   41
                                   SCHEDULE C







                                SEE EXHIBIT 10.12
   42
                        [MERCK & CO., INC. - LETTERHEAD]



                                  SCHEDULE D



May 29, 1991

Mr. Kenneth U. Johnson
Marketing-Licensing
Wisconsin Alumni Research Foundation
P.O. Box 7365
Madison, Wisconsin 53707-7365

          Re:  Research Collaboration and License Agreement
               Between Vical Incorporated and Merck & Co., Inc.

Dear Mr. Johnson:

Vical Incorporated and Merck & Co., Inc., anticipate entering into a Research
Collaboration and License Agreement ("Vical License Agreement") involving
intramuscular delivery of vaccines for the prevention of human infectious
disease ("the Technology"). Under the proposed Agreement Merck would be granted
an exclusive license from Vical.

Merck is aware that certain inventions involving the Technology are jointly
owned by Vical and WARF and that pursuant to a License Agreement between the
parties, dated January 1, 1991, ("WARF License Agreement") WARF has granted
Vical an exclusive License to its interest in certain of the Technology. The
grant is subject only to the rights, if any, of the United States Government to
practice the invention for government purposes.

Pursuant to Section 2C of the WARF License Agreement, Merck hereby requests
WARF's consent permitting Vical to extend the exclusive license granted by Vical
to Merck with respect to WARF's ownership interest in certain of the Technology
beyond any termination of the WARF License Agreement. In the event of such
termination MERCK agrees to be bound by the terms of the WARF License Agreement
to the extent of its license from Vical and for so long as the Vical License
Agreement remains in affect.
   43
                                     - 2 -


If WARF is willing to permit such extension please have WARF indicate its
consent by having an authorized representative sign and date this letter in the
space provided below and return one original to me. 

We have asked Vical to acknowledge its agreement to this arrangement by also
signing and dating this letter. 

Your cooperation in this matter is appreciated.

Sincerely,

/s/ George W. Brodhead
- - ---------------------------------
George W. Brodhead

5733n
/ac


AGREED TO AND ACCEPTED:

WISCONSIN ALUMNI RESEARCH
     FOUNDATION

By: /s/ John R. Pike
    -----------------------------

Date:  5/31/91
      ---------------------------

ACKNOWLEDGED:

VICAL INCORPORATED



By:  /s/ Dannie H. King
    -----------------------------

Date:  6/6/91
      ---------------------------
approved for execution:

/s/ P. Jeffrey Archibald
- - ---------------------------------
P. Jeffrey Archibald

May 31, 1991