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                                                                   Exhibit 10.52
Searle
Box 5110
Chicago, Illinois 60680-5110
Telephone 847 982 7000
Fax 847 470 1480

September 5, 1997                                                   CONFIDENTIAL



Cytel Corporation
3525 John Hopkins Court
San Diego, CA 92121

        Attention:    Virgil D.  Thompson
                      President and CEO

Gentlemen and Ladies:

G.D. Searle & Co. ("Searle"), a wholly-owned subsidiary of Monsanto Company
("Monsanto"), is pleased to submit an offer to enter into and complete a
transaction which would result in an equity investment in Cytel Corporation
("Cytel") and in certain licensing and other collaborative activities, all as
more particularly described herein. Searle is extremely interested in
establishing a collaborative alliance with Cytel to maximize its technology in
the cancer treatment area and believes our offer demonstrates our commitment to
do so.

I.      OFFER. SUBJECT TO THE CONDITIONS CONTAINED IN THIS OFFER LETTER, SEARLE
        OFFERS TO MAKE AN EQUITY INVESTMENT IN CYTEL IN THE TOTAL AMOUNT OF ***
        IN THREE (3) PHASES, THE DETAILS OF WHICH ARE DESCRIBED BELOW. IN
        CONSIDERATION FOR SUCH *** SEARLE WILL RECEIVE AN EQUITY POSITION IN
        CYTEL AND CERTAIN LICENSE RIGHTS TO CYTEL TECHNOLOGY AS DESCRIBED UNDER
        PHASE I AND PHASE II BELOW. UPON CYTEL'S ACCEPTANCE OF THE CONDITIONS
        CONTAINED IN THIS OFFER LETTER, SEARLE IS PREPARED TO IMPLEMENT PHASE I.

        A.  Phase I

            1.  Equity Investment.  Within seven (7) business days after Cytel's
                acceptance of this offer, Searle will purchase shares of common
                stock of Cytel from Cytel at a purchase price equal to $2.50 per
                share, *** as part of the current private placement or, if such
                private placement is terminated for whatever reason or completed
                at a price lower than $2.00 per share, the number of shares
                purchased by Searle shall be increased so that the average
                purchase price of the shares is the higher of $2.25 or $0.50
                above the private placement price for an aggregate purchase
                price of five million


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                dollars ($5,000,000) provided (a) *** of the issued and
                outstanding shares of common stock of Cytel on the date of
                purchase by Searle, (b) Cytel shall use the proceeds of the sale
                of the shares for the purpose of funding its Epimmune(TM) unit,
                and (c) Cytel indicates in writing its agreement to the
                provisions of this Section I.A. and to the concepts contained in
                the remainder of this offer letter which shall be incorporated
                into definitive agreements to be negotiated in good faith by
                Cytel and Searle during the Exclusivity Period (defined in
                paragraph I.A.3). The shares will be purchased by Searle in
                accordance with the terms and conditions of a Stock Purchase
                Agreement substantially in the form of Exhibit A attached
                hereto. The parties acknowledge that any sale by Searle of Cytel
                common stock will be pursuant to Rule 144 of the Securities Act
                of 1983.

            2.  License Rights.
                (a)  Cytel will grant to Searle under the definitive agreements
                     *** to make, have made, use and sell pharmaceutical
                     products derived from Cytel's cancer epitopes and *** in
                     accordance with the terms and conditions (including
                     appropriate *** of a license agreement mutually acceptable
                     to the parties. With respect to *** Cytel grants to Searle
                     ***
                (b)  If by the end of the Exclusivity Period, the parties have
                     not agreed on the terms and conditions of such license
                     agreement, then, except as otherwise provided in this
                     subsection (b) *** to obtain *** on the same terms and
                     conditions as contained in *** Cytel shall provide a copy
                     of *** The rights granted in this subsection (b) shall not
                     apply in


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                the event Searle does not execute definitive agreements due to
                the reasons stated in Section III.1.

            3.  Exclusivity. Subject to the completion of the equity investment
                under Section I.A.1. beginning *** and continuing *** (the
                "Exclusivity Period"), Cytel shall not directly or indirectly
                *** During the Exclusivity Period, Searle and Cytel shall
                negotiate in good faith the definitive agreements contemplated
                hereunder.

        B.  Phase II - Equity Investment.

        Upon execution of the definitive agreements contemplated hereunder,
        Searle will purchase *** of the common stock of Epimmune Entity (as
        defined in Section I.D.9), for the purpose of funding the Epimmune
        Entity; provided, however, *** In such event, *** Such *** (a) *** at a
        time and price to be agreed, or (b) *** or (c) *** For the avoidance of
        doubt, *** The per share purchase price of the common stock of Epimmune
        shall be agreed by the parties based on ***


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        ***  first set forth above in this Section B  ***

        C.  Phase III - Equity Investment.

        Not later than *** Searle will purchase *** of the common stock of
        Epimmune Entity (as defined in Section I.D.9), for the purpose of ***
        provided, however, *** In such event, *** Such *** (a) *** at a time and
        price to be agreed, or (b) *** or (c) *** For the avoidance of doubt,
        *** The per share purchase price of the common stock of Epimmune shall
        be agreed by the parties based on ***

        D.  Related Covenants

            1.  License Rights. Cytel will grant to Searle *** to make, have
                made, use and sell pharmaceutical cancer therapy products *** in
                accordance with the terms and conditions (including appropriate
                *** of a license agreement mutually acceptable to the parties.

            2.    ***  to be

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                agreed upon by the parties in the definitive agreements
                contemplated hereunder. In addition, in the event ***

            3.    ***

            4.  *** promptly following execution of the definitive agreements
                contemplated hereunder.

            5.  Development of Products.
                (a)  The parties will collaborate and agree on a development
                     plan for the products licensed pursuant to paragraphs
                     I.A.2. and I.D.1. *** (the "Products"). The pre-clinical
                     and early clinical development of the Products shall be the
                     responsibility of Cytel. Searle shall *** as mutually
                     agreed by the parties.
                (b)  The pre-clinical and clinical development of the Products
                     shall be managed by a Joint Management Committee composed
                     of an equal number of representatives from each party. The
                     Joint Management Committee shall approve and oversee
                     implementation of the development plans for the Products.
                     If the Joint Management Committee is unable to reach
                     consensus on any issue for which it has responsibility ***


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                (c)  In connection with the license agreements referred to in
                     paragraphs I.A.2. and I.D.1 *** the parties will negotiate
                     *** referred to in this paragraph I.D.5. *** The
                     circumstances constituting *** shall be agreed upon in the
                     definitive agreements.

            6.  Provision of Services. Cytel and Searle shall enter into an
                agreement pursuant to which Searle will provide services for ***
                pursuant to paragraphs I.A.2., I.D.1 *** Searle will provide
                such services *** to be agreed by the parties.

            7.  Key Employees. If, in the course of conducting its due diligence
                investigation of Cytel, Searle determines any Cytel employees
                are critical to the development of the Products, Searle and
                Cytel shall discuss reasonable methods for securing the
                employment of such employees.

            8.  Preemptive Rights. Any purchase by Searle, as contemplated by
                this offer letter, of the common stock of Cytel *** shall be
                accompanied by the grant to Searle of preemptive rights to be
                agreed by the parties.

            9.  Epimmune Entity. Cytel will undertake to form a separate entity
                for its Epimmune unit ("Epimmune Entity"), contribute its assets
                and liabilities relating to the Epimmune unit and transfer
                appropriate personnel necessary to continue the operations of
                the Epimmune unit. In addition, Cytel shall ***

II.     TIMING. WE UNDERSTAND THAT CYTEL HAS A STRONG INTEREST IN CONCLUDING A
        TRANSACTION OF THIS NATURE AS EXPEDITIOUSLY AS POSSIBLE. WE BELIEVE THAT
        SEARLE IS A PREFERRED PARTNER IN THIS

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        REGARD. WE ARE COMMITTED TO A SPEEDY TRANSACTION CLOSE. INDEED, WITH THE
        COOPERATION OF CYTEL, OUR AIM IS TO CLOSE THE COMPLETE TRANSACTION 
        CONTEMPLATED IN SECTIONS I.B.I.C. AND I.D. AS SOON AS POSSIBLE AFTER ***

III.    CONDITIONS OF OFFER.  Searle's offer and the consummation of the 
        transactions contemplated in Sections I.B., I.C. and I.D hereunder are 
        subject to the following terms and conditions:

            1.  obtaining all necessary corporate authority to conclude the 
                transactions contemplated hereunder;

            2.  completion of and satisfaction with *** as deemed necessary by
                Searle including but not limited to *** Cytel warrants it will
                disclose all such information;

            3.  execution of mutually agreed upon definitive agreements
                detailing the terms of the transactions contemplated in Sections
                I.B, I.C. and I.D. hereunder and containing such other
                representations and warranties, indemnities, covenants,
                conditions and provisions customarily contained in agreements
                for transactions of similar nature and size as the transactions
                contemplated hereunder;

            4.  the expiration, without any adverse governmental action, of any 
                applicable waiting periods under the Hart-Scott-Rodino Antitrust
                Improvements Act of 1976;

            5. receipt of Cytel's written acceptance referred to in paragraph
               I.A.l.(c).

           Nothing in this offer shall be construed in any way or for any reason
           to obligate Searle to proceed with Phase II and/or Phase III. If
           Searle decides not to proceed with Phase II and/or III, Searle shall
           not be liable to Cytel in any amount and/or for any reason based on
           this offer. This offer be amended only in writing signed by both
           parties.


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IV.     MISCELLANEOUS.  Except with respect to the investment contemplated in 
        Section I.A.1, this offer does not contain all matters upon which
        definitive agreements must be reached and is expressly conditioned upon
        Searle and Cytel entering into mutually satisfactory definitive
        agreements, in form and substance satisfactory to their respective
        counsel. Except with respect to the investment contemplated in Section
        I.A.1, nothing contained in this offer letter shall obligate Searle to
        proceed with the transactions contemplated hereunder if all of the
        conditions listed in Section III of this letter are not met.

        Cytel and Searle will keep this offer letter, its attachments and the
        negotiations relating to this transaction confidential and will not
        disclose (except as required by law) the terms of the offer, the
        definitive agreements or the negotiations between the parties to any
        third party without the express written consent of the other.

        Searle acknowledges that Cytel will be obligated to announce the
        acceptance of this offer, the equity investment completed under Section
        I2A.1 and the scope of the transaction contemplated hereunder, excluding
        specific terms and conditions. Such announcement will be agreed to by
        the parties and issued immediately upon acceptance of this offer.

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Cytel Corporation
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If the foregoing offer is acceptable to Cytel, please acknowledge your
acceptance of the offer and provisions set forth above by signing this letter in
duplicate below and returning one original to Searle no later than 5:00 p.m.,
Central Standard Time on September 5, 1997. This offer expires as of that date
and time if it is not accepted in the manner provided herein.

Searle is committed to forging an alliance with Cytel that is beneficial to both
parties and we are, therefore, willing to work with Cytel to reach a transaction
structure which meets both our business objectives. If you have any questions
about this offer or need further clarification of any item, please contact ***

Sincerely,                                     Accepted and Agreed:

G.D. Searle & Co.                              CYTEL CORPORATION

By: /s/ Richard U. De Schutter                 By: /s/ Virgil Thompson
   -----------------------------                  ------------------------------
    Richard U. De Schutter
    Chief Executive Officer                    Title: President & CEO
                                                      --------------------------
                                               Date:  September 5, 1997
                                                      --------------------------
cc:  Deborah A.  Schueren
     V.P. Finance & CFO