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                                                              Exhibit 10.15
                                                              -------------

                                Merck & Co., Inc.

                   Amendment to Conditions of Employment

          All employees, upon acceptance of employment with Merck & Co., Inc.
(the "Company"), are required to sign the Company's standard "Conditions of
Employment" agreement regarding matters such as confidentiality, ownership of
inventions, and Company policies and regulations. A copy of the Conditions of
Employment agreement you signed is attached for your reference. Employees of
Merck-Medco Managed Care, L.L.C. (with its subsidiaries, collectively "Medco")
enter into agreements which contain non-competition, non-solicitation, and other
undertakings. As a result of the merger of Medco with Merck, and in
consideration of your having been exposed to, and in the future being exposed
to, confidential information concerning Medco and the "Business" as defined
below, it is necessary to request Merck employees who work for Medco to also
agree to non-competition and non-solicitation provisions. Accordingly, the
following paragraphs 8, 9, 10 and 11 will be incorporated into and made part of
your "Conditions of Employment" agreement upon your signature to this Amendment:

          8.   During my employment at Medco and for a period of one (1)
               year after the termination of such employment, for any
               reason, absent the Company's express prior written approval,
               I will not (as an individual, principal, agent, employee,
               consultant, or otherwise), directly or indirectly, in any
               territory that Medco does business and/or markets its
               products and services, engage in any activities that are
               competitive with, nor render services to any firm or
               business engaged or about to become engaged in competition
               with Medco, which includes, but is not limited to: (i) the
               third party prescription drug claims processing business;
               (ii) the marketing of or consulting as to prescription drug
               benefit plans; (iii) the pharmacy benefit management or
               disease management businesses; (iv) the organization and
               administration of retail pharmacy networks; (v) the
               provision of prescription drugs through mail service; and/or
               (vi) any other business in which Medco is then engaged as to
               which I have acquired confidential information (hereinafter
               collectively the "Business"). In addition, I will not have
               an equity interest in any such firm or business other than
               as a 1% or less shareholder of a public corporation. This
               Agreement does not prevent my employment with a prescription
               pharmaceutical company, a part of which is engaged in the
               Business, so long as my responsibilities do not include any
               activity competitive with the Business, and I so certify to
               Medco in writing on a quarterly basis during the one-year
               term specified above.

          9.   During my employment at Medco and for a period of two (2) years
               after the termination of such employment, for any reason, I will
               not, on behalf of myself or any other person, firm, or
               organization, directly or indirectly: (i) induce, or attempt to
               induce, any employees, agents, or consultants of Medco or the
               Company to do anything from which I am restricted by reason
               hereof; (ii) offer or aid others to offer employment to any
               employees, agents or consultants of Medco or the Company who are
               involved in the Business; and/or (iii) contact or solicit any of
               Medco's or the Company's customers or targeted potential
               customers for the purpose of offering products or services that
               directly or indirectly compete or interfere with the Business.

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          10.  (a) In consideration of my agreement and the restrictions
               contained in this Amendment to the "Conditions of Employment"
               agreement, the Company agrees: (i) if, while I am employed at
               Medco, the Company terminates my employment for reasons other
               than cause and I am not then eligible for retirement from active
               service, I shall be entitled to twelve (12) monthly payments at
               my then monthly base rate, subject to withholding and any
               deductions for benefit coverage. For purposes of this Agreement,
               I will be deemed an inactive employee until the earlier of the
               expiration of such monthly payment period or until I obtain
               employment elsewhere. During such inactive employee status, I may
               continue to participate in any benefit plans in which I
               participated at the time of my termination other than any
               disability plans; (ii) if, while I am employed at Medco, the
               Company terminates my employment for reasons other than cause and
               I retire from active service, then I shall be entitled to the
               above number of monthly payments at my then monthly base rate,
               subject to withholding; (iii) should I die prior to receiving all
               monthly payments to which I am entitled to hereunder, any
               remaining payments shall be paid in a lump sum to my estate.

               (b) This Agreement shall be construed in accordance with and
               governed for all purposes by the laws and public policy of New
               Jersey, without regard to principles of conflict of laws. If I
               breach any of the provisions of the "Conditions of Employment"
               agreement (including this Amendment), then, in addition to any
               other rights or remedies of the Company, the Company shall have
               no obligation to make any monthly payments to me and any status
               as an inactive employee will then terminate immediately.

          11.  The term "Company" as used in the "Conditions of Employment"
               agreement (including this Amendment) shall be deemed to include
               Medco.

          The provisions of paragraphs 8 and 9 shall be construed, and limited
or reduced if necessary, by a court so as to be enforceable to the maximum
extent permitted by applicable law and, without limiting any remedy available,
shall be enforceable by injunctive or equitable relief.

          Please sign and return a copy of this Amendment confirming your
agreement hereto.

                                /s/ Richard Clark

                                --------------------------------------
                                  Richard Clark


                                January 29, 1999

                                --------------------------------------
                                Date

Enclosure

                                /s/ Bert Weinstein

                                --------------------------------------
                                Signature of Company Representative

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                             MERCK & CO., INC.

                            CONDITIONS OF EMPLOYMENT

Prepare in Duplicate

          The purpose of this statement is to define the conditions which a new
employee accepts when he joins Merck & Co., Inc. ("the Company"). The employee's
signature indicates that he understands this statement and agrees to abide by
it. A copy is provided to the new employee in order to avoid any possible
misunderstandings regarding the provisions of this agreement.

          1.   I will abide by all the policies and regulations of the
               Company, will subscribe to the Company's Group Life
               Insurance Plan and will join and qualify under the Company's
               Retirement Income Plan as soon as I am eligible under the
               terms of the Plan. I have received and read booklets
               describing the Group Life Insurance Plan and the Company's
               Retirement Income Plan. I also agree that I will be subject
               to Workmen's Compensation Laws of the state or states in
               which I will be employed by the Company.

          2.   I will devote my best efforts to the service of the Company, will
               perform such duties as may be assigned to me and will not engage
               in other employment and/or activities that conflict with or
               impair my obligations as an employee of the Company.

          3.   I will not, during or at any time after the period of my
               employment by the Company, use for myself or others or
               divulge or convey to others any information, knowledge, data
               or property relating to the Company's business in any way
               obtained by me while employed by the Company other than
               published material properly in the public domain unless
               authorized by the Company in writing or by established
               Company procedures. This includes but is not limited to
               information, knowledge, data or property concerning any
               process, apparatus or product manufactured, used, developed,
               investigated or considered by the Company.

          4.   All memoranda, notes, records, papers or other documents
               (and all copies thereof) relating to the Company's business
               and all property associated therewith (such as but not
               confined to organisms, compounds and models) in any way
               obtained by me while employed by the Company shall be the
               Company's property and shall be delivered by me to the
               Company on termination of employment or at any time on the
               Company's request together with my written certification of
               compliance. This includes but is not limited to such
               documents and property concerning any process, apparatus or
               product manufactured, used, developed, investigated or
               considered by the Company.

          5.   All inventions or discoveries relating to the Company's business
               which I conceive or make while employed by the Company shall be
               the Company's property. Any invention

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               relating to the Company's business disclosed by me within one
               year following the termination of this employment shall be deemed
               to fall within this provision, unless proved to have been
               conceived and made following such termination.

          6.   When requested by the Company, whether during or subsequent
               to this employment, I will execute patent applications and
               other instruments considered necessary by the Company to
               apply for and obtain Letters Patent of the United States and
               foreign countries covering such inventions or discoveries
               and will make assignments and execute other instruments
               necessary to convey to the Company ownership and exclusive
               rights in such inventions, discoveries, patent applications
               and patents.

          7.   The Company shall bear all expenses connected with such patents,
               patent applications and maintenance of patent protection, and if
               services in connection therewith are performed by me at the
               request of the Company after termination of this employment, the
               Company shall pay reasonable compensation for such
               post-employment services.

Signature of New Employee /s/ Richard T. Clark, Jr.     Date August 14, 1972
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                            Reports to D. O. Kennedy

Signature of Company Representative /s/ H. J. Hurst     Date August 14, 1972
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