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                                                                   Exhibit 10.45

                                   GUARANTEE


     This GUARANTEE is dated as of October 29, 1998 by Schering-Plough
Corporation, a New Jersey corporation ("GUARANTOR") in favor of Pharmacopeia,
Inc., a Delaware corporation ("BENEFICIARY").

     WHEREAS, Schering Corporation, a New Jersey corporation ("SCHERING") and
Beneficiary have entered into a Collaboration and License Agreement (the
"SCHERING AGREEMENT") dated as of October 29, 1998;

     WHEREAS, Schering-Plough Ltd., a Swiss corporation ("LTD.") and Beneficiary
have entered into a Collaboration and License Agreement (the "LTD. AGREEMENT")
dated as of October 29, 1998 (Schering and Ltd. are collectively referred to as
the "SUBSIDIARIES") (the Schering Agreement and the Ltd. Agreement are
collectively referred to as the "AGREEMENTS");

     WHEREAS, Guarantor is the parent company of the Subsidiaries; and

     WHEREAS, to induce Beneficiary to enter into the Agreements, and for other
good and valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the Guarantor has agreed to guarantee the Guaranteed Obligations
(as hereinafter defined) on the terms set forth herein.

     NOW THEREFORE, the parties hereto agree as follows:

     Section 1.  Guarantee.  Subject to the terms and conditions set forth
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below, Guarantor hereby irrevocably and unconditionally guarantees the
performance of all obligations of the Subsidiaries under the Agreements and
promises to pay in full any amounts, including any arbitration awards, which
become due by either or both of the Subsidiaries to Beneficiary under the
Agreements, except to the extent previously paid by the Subsidiaries (such
obligations being herein collectively called the "GUARANTEED OBLIGATIONS" or
"GUARANTEED PAYMENTS").  Guarantor's obligation to make Guaranteed Payments may
be satisfied by direct payment of the required amounts to Beneficiary or by
causing a Subsidiary to pay such amounts to Beneficiary.

                 1.1   Waiver of Certain Rights. Guarantor hereby waives, to the
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fullest extent permitted by law, notice of acceptance of this Guarantee and of
any liability to which it applies or may apply, presentment, demand for payment,
protest, notice of nonpayment, notice of dishonor, notice of redemption and all
other notices and demands.

                 1.2   Continuing Guarantee.  The guarantee in this Section 1 is
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a continuing guarantee, and shall apply to all Guaranteed Obligations whenever
arising.

                 1.3   Adequate Information.  Guarantor acknowledges and
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confirms that Guarantor itself has established its own adequate means of
obtaining from its Subsidiaries on a continuing basis all information desired by
Guarantor concerning the financial condition of the 
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Subsidiaries and their performance under the Agreements and that Guarantor will
look to the Subsidiaries and not to Beneficiary in order for Guarantor to keep
adequately informed of changes in the Subsidiaries' financial condition or
performance under the Agreements.

                 1.4   Proceeding Directly Against Guarantor.  This Guarantee is
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a guarantee of payment and not of collection. Beneficiary may enforce this
Guarantee directly against Guarantor, and Guarantor waives any right or remedy
to require that any action be brought against the Subsidiaries or any other
person or entity before proceeding against Guarantor. All waivers herein
contained shall be without prejudice to the right of Beneficiary, at its option,
to proceed against the Subsidiaries, whether by separate action or by joinder.
Guarantor agrees that this Guarantee shall not be discharged except by payment
of the Guarantee Payments in full (to the extent not previously paid by the
Subsidiaries) and by complete performance of all Guaranteed Obligations under
this Guarantee.

                 1.5   Independent Obligations.  Guarantor acknowledges that its
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obligations hereunder are independent of the obligations of the Subsidiaries
with respect to the Agreements and that Guarantor shall be liable as principal
and sole debtor under this Guarantee to make Guarantee Payments in full pursuant
to the terms of this Guarantee.

                 1.6   Amendment of Agreements Not Require Guarantor Consent.
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Any amendment, modification or alteration of, or waiver by the Subsidiaries with
respect to the terms of the Agreements shall be effective without requiring any
consent on the part of the Guarantor.

     Section 2.  Representations and Warranties.  Guarantor represents and
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warrants to Beneficiary that (a) Guarantor is a corporation, duly organized,
validly organized and in good standing under the laws of the State of New
Jersey, (b) Guarantor has all requisite power and authority to enter into and
perform this Guarantee and all corporate action on the part of Guarantor
necessary for the execution and performance of this Guarantee has been taken,
(c) this Guarantee is the legal and binding obligation of Guarantor, enforceable
in accordance with its terms, except where such enforceability may be limited by
bankruptcy, insolvency, or other laws of general application relating to or
affecting the enforcement of creditors' rights generally, and (d) Guarantor's
execution, delivery and performance of this Guarantee will not violate or
conflict with any laws, regulations or agreements binding upon Guarantor or to
which Guarantor is otherwise subject or is a party.

     Section 3.  Miscellaneous.
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                 3.1   Governing Law.  This Guarantee shall be governed by, and
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construed in accordance with, the laws of the State of New Jersey.

                 3.2   Successors and Assigns.  This Guarantee may not be
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assigned or transferred by Guarantor without the express written consent of
Beneficiary. This Guarantee shall be binding upon Guarantor, its permitted
successors and assigns, and shall inure to the benefit of and be enforceable by
Beneficiary and its successors, transferees and assigns.

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                 3.3   Attorneys' Fees.  If Beneficiary commences any action or
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proceeding against Guarantor to enforce this Guarantee, the prevailing party in
such action or proceeding shall be entitled to recover from the other party the
reasonable attorneys' fees and other costs and expenses incurred by that
prevailing party in connection with such action or proceeding and in connection
with enforcing any judgment, award or order thereby obtained.

                 3.4   Miscellaneous.  This Guarantee, together with the
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Agreements, constitutes the entire agreement of Guarantor with respect to the
matters set forth herein. The rights and remedies herein provided are cumulative
and not exclusive of any remedies provided by law or any other agreement, and
this Guarantee shall be in addition to any other guarantee of or collateral
security provided by the Subsidiaries or any other person or entity for any of
the Guaranteed Obligations. The invalidity or unenforceability of any one or
more sections of this Guarantee shall not affect the validity or enforceability
of its remaining provisions. Captions are for ease of reference only and shall
not affect the meaning of the relevant provisions. The meanings of all defined
terms used in this Guarantee shall be equally applicable to the singular and
plural forms of the terms defined.

                 3.5   Termination.  This Guarantee shall terminate
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automatically upon the termination of both the Ltd. Agreement and the Schering
Agreement.

     IN WITNESS WHEREOF, the undersigned does hereby execute this Guarantee as
of the date first set forth above.

                                  "GUARANTOR"                                
                                                                             
                                                                             
                                  By: /s/ Raul Cesan                        
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                                  Name: Raul Cesan                       
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                                  Title: Corporate Executive Vice President
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