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                                                                   EXHIBIT 10.30

                                                                    May 16, 2003

Mr. Gabe Leung
125 Top of the Way World
Green Brook, NJ  08812

         Re:      Employment Agreement

Dear Gabe:

         This letter is to confirm our understanding with respect to (i) your
future employment by OSI Pharmaceuticals, Inc. (the "Company"), (ii) your
agreement not to solicit employees or customers of the Company, or any present
or future parent, subsidiary or affiliate of the Company (each, a "Company
Affiliate" and collectively, together with the Company, the "Company," (iii)
your agreement to protect and preserve information and property which is
confidential and proprietary to the Company, and (iv) your agreement with
respect to the ownership of inventions, ideas, copyrights and patents which may
be used in the business of the Company (the terms and conditions agreed to in
this letter are hereinafter referred to as the "Agreement"). In consideration of
the mutual promises and covenants contained in this Agreement, and for other
good and valuable consideration, the receipt and sufficiency of which are hereby
mutually acknowledged, we have agreed as follows:

         1.       Employment.

                  (a)      Subject to the terms and conditions of this
         Agreement, the Company will employ you, and you will be employed by the
         Company and/or any Company Affiliate designated by the Company,
         initially as Executive Vice President and President, Oncology Business,
         of the Company reporting to Chief Executive Officer (the "CEO") of the
         Company. You will have the responsibilities, duties and authority
         customarily performed, undertaken and exercised by a person in a
         similar executive capacity. You will also perform such other and/or
         different services for the Company as may be assigned to you from time
         to time by the CEO. The principal location at which you will perform
         such services will be the Company's headquarters located at 58 South
         Service Road, Melville, New York, although you will be available to
         perform services at any other Company facility and to travel as the
         needs of business may require.

                  (b)      Devotion to Duties. While you are employed hereunder,
         you will, to the best of your ability, perform faithfully and
         diligently all duties assigned to you pursuant to this Agreement and
         will devote your full business time and energies to the-business and
         affairs of the Company. While you are employed hereunder, you will not
         undertake any other employment from any person or entity without the
         prior written consent of the Company.

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         2.       Term. Except for earlier termination as provided for in
         Section 4 hereof, your employment under this Agreement (the "Employment
         Term") shall be for an initial term commencing on May 21, 2003 (the
         "Effective Date") and ending on the third anniversary of the Effective
         Date (the "Initial Term"). Unless written notice is given of an intent
         not to extend the Initial Term or any extension thereof by you or the
         Company at least 90 days prior to an anniversary of the Effective Date,
         the Employment Term shall be deemed, as of such 90th day, to have been
         extended and continue until the end of the successive 12-month period
         unless otherwise terminated as provided for in Section 4 hereof. In the
         event the Company elects not to renew the Employment Term, you shall be
         entitled to the payments and benefits set forth in Section 6(c).

         3.       Compensation.

                  (a)      Base Salary. While you are employed hereunder, the
         Company will pay you a base salary at the annual rate of $350,000 (the
         "Base Salary"). Your Base Salary will be reviewed on an annual basis
         each September (or such other time as determined by the CEO or the
         Compensation Committee of the Board of Directors of the Company (the
         "Board")), commencing with September, 2004. The Base Salary will be
         payable in equal installments in accordance with the Company's payroll
         practices as in effect from time to time. The Company will deduct from
         each such installment all amounts required to be deducted or withheld
         under applicable law or under any employee benefit plan in which you
         participate.

                  (b)      Bonus. In addition to the Base Salary, for each
         fiscal year of the Company ending during the Term of the Agreement,
         beginning with the 2004 fiscal year, you will be eligible to receive a
         target bonus of up to 50% of the Base Salary, determined and payable in
         accordance with the Company's practices applicable to bonuses paid to
         its executives. The Company's bonus system is a discretionary annual
         performance-based incentive bonus system, approved by the Company's
         Board, and is based upon a combination of personal and corporate
         performance contributing to your maximum target. Bonuses are determined
         in December of each year. For fiscal year 2003, you will receive a
         guaranteed bonus equal to 30% of your Base Salary, or $105,000, payable
         in December 2003 or January 2004, at your option.

                  (c)      Equity Compensation.

                           (i)      Initial Grant. On the Effective Date and
                  pursuant to a written stock option agreement (the "Stock
                  Option Agreement") between the Company and you under the OSI
                  Pharmaceuticals, Inc. 2001 Incentive and Non-Qualified Stock
                  Option Plan (the "Plan"), you will be granted a non-qualified
                  option (the "Initial Option") to purchase 150,000 shares of
                  the Company's common stock, par value, $.01 per share (the
                  "Common Stock"). The exercise price for the Initial Option
                  will be the fair market value per share of the Common Stock on
                  the date the Initial Option is

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                  granted and the other terms and conditions of the Initial
                  Option will be as set forth in the Plan and the Stock Option
                  Agreement. The Initial Option will have a term of 10 years and
                  will become exercisable with respect to 33% of such shares on
                  the first anniversary of the date of grant with the remaining
                  shares vesting over the succeeding 24 months following the
                  first anniversary of the date of grant on a monthly pro-rated
                  basis. Notwithstanding the foregoing, the Initial Option shall
                  vest and be fully exercisable upon a Change of Control (as
                  hereinafter defined).

                           (ii)     Future Grants. On each date that annual
                  stock options are granted by the Company to its executive
                  management group, so long as you then remain in the employ of
                  the Company, the Company will grant to you an option (an
                  "Annual Option") to purchase a number of shares of Common
                  Stock to be determined by the Compensation Committee of the
                  Board based upon your grade level. The target for grade 11
                  ranges from 35,000 to 40,000, subject to the discretion of the
                  Compensation Committee of the Board. The exercise price for
                  each Annual Option will be the fair market value per share of
                  Common Stock on the date the Annual Option is granted and the
                  other terms and conditions of the Annual Option will be as set
                  forth in the Plan and Option Agreement accompanying such
                  Annual Option. Each Annual Option will have a term of 10 years
                  and will become exercisable with respect to 33% of such shares
                  on the first anniversary of the date of grant with the
                  remaining shares vesting over the succeeding 24 months
                  following the first anniversary of the date of grant on a
                  monthly pro-rated basis. Notwithstanding the foregoing, each
                  Annual Option shall vest and be fully exercisable upon a
                  Change of Control (as hereinafter defined).

                  (d)      Vacation. You will be entitled to 22 paid vacation
         days in each calendar year, including 2003, and paid holidays plus
         personal days in accordance with the Company's policies for its senior
         executives as in effect from time to time.

                  (e)      Fringe Benefits. In addition to the equity
         compensation provided for herein, you will be entitled to participate
         in employee benefit plans which the Company provides or may establish
         for the benefit of its senior executives generally (for example, term
         life, disability, medical, dental and other insurance, retirement,
         pension, profit-sharing and similar plans) (collectively, the "Fringe
         Benefits"). Your eligibility to participate in the Fringe Benefits and
         receive benefits thereunder will be subject to the plan documents
         governing such Fringe Benefits. Nothing contained herein will require
         the Company to establish or maintain any Fringe Benefits.

                  (f)      Company Car. The Company will provide you with an
         approved company car and will reimburse you for related expenses in
         accordance with the Company's policy for other similarly situated
         senior executives.

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                  (g)      Relocation. The Company will assist you in your
         relocation to Long Island in accordance with the following:

                           (i)      Relocation Service. Cendant Mobility will
                  assist you in your relocation from New Jersey to Long Island
                  in accordance with the Company's agreement with Cendant
                  Mobility.

                           (ii)     Current Residence. The Company, through
                  Cendant Mobility and at your request, will purchase your
                  current residence in New Jersey for a purchase price equal to
                  its fair market value as determined by the Company's agreement
                  with Cendant Mobility.

                           (iii)    Expenses. The Company will reimburse you,
                  either directly or through its relocation service, Cendant
                  Mobility, for all expenses relating to your relocation from
                  New Jersey to Long Island, including, without limitation,
                  expenses relating to packing and moving household goods,
                  temporary storage of household goods, closing costs (including
                  mortgage points) associated with the purchase of a new home on
                  Long Island, expenses incurred by you and/or your family
                  relating to house-hunting trips (including transportation,
                  hotel accommodations and meals) and expenses incurred by you
                  and/or your family for temporary living accommodations for up
                  to three months prior to your move into a new home (including
                  transportation, lease or sublease amounts, hotel or other
                  accommodations, meals, brokers' fees).

                           (iv)     Mortgage Assistance. For a period of three
                  years following your purchase of a home on Long Island (the
                  "Long Island Home"), the Company will pay you a mortgage
                  assistance allowance equal to $50,000 per annum, payable in
                  equal monthly installments. The foregoing is subject to your
                  continued employment with the Company during such three-year
                  period. If (A) your employment terminates "without cause" (as
                  defined in Section 4(e) hereof), (B) you terminate your
                  employment for "good reason" (as defined in Section 4(d)
                  hereof), or (C) upon a "Change of Control" (as defined in
                  Section 7 hereof) prior to the end of such three-year period,
                  you will continue to receive such mortgage assistance
                  allowance on the terms described in this section provided that
                  you remain resident in the Long Island Home.

                           (v)      Lump Sum Payment. In addition to the amounts
                  provided for in Sections 3 (g) (iii) and (iv), upon your
                  purchase of the Long Island Home, you will receive a one-time
                  lump sum payment equal to $25,000 to cover incidental
                  expenses.

                           (vi)     Gross-Up. In addition to the amounts set
                  forth in Sections 3 (g) (iii), (iv) and (v), you will be
                  entitled to receive an additional payment (a "Gross-Up
                  Payment") in an amount equal to all

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                  taxes (including any interest or penalties imposed with
                  respect to such taxes) paid by you with respect to such
                  amounts.

                           (vii)    Pay-Back. If, within 18 months following the
                  Effective Date, you terminate your employment with the Company
                  "without good reason" (as defined in Section 4(f) hereof), you
                  shall pay back a pro-rated portion (based upon the date of
                  your termination) of the amounts paid to you pursuant to
                  Sections 3 (g) (iii), (iv), (v) and (vi) (the "Relocation
                  Expenses") in accordance with the following schedule:

                           (A)      If such termination occurs within 6 months
                  of the Effective Date hereof, you will be responsible to repay
                  100% of the Relocation Expenses;

                           (B)      If such termination occurs between 7 to 12
                  months following the Effective Date, you will be responsible
                  to repay 75% of the Relocation Expenses; and

                           (C)      If such termination occurs between 13 to 18
                  months following the Effective Date, you will be responsible
                  to repay 50% of the Relocation Expenses.

                           (viii)   Documentation. Reimbursement of the expenses
                  provided for in this Section 3(g) shall be made upon
                  presentation of documentation reasonably satisfactory to the
                  Company in accordance with the Company's policies with respect
                  thereto as in effect from time to time. Receipts shall not be
                  required for payment covered under Section 3 (g) (v).

                  (h)      Reimbursement of Expenses. Upon presentation of
         documentation of such expenses reasonably satisfactory to the Company,
         the Company will reimburse you for all ordinary and reasonable
         out-of-pocket business expenses that are reasonably incurred by you in
         furtherance of the Company's business in accordance with the Company's
         policies with respect thereto as in effect from time to time.

         4.       Termination. The Employment Term shall end upon the earliest
         of the following to occur:

                  (a)      Your death.

                  (b)      Upon written notice to you of termination as a result
         of your Permanent Disability. "Permanent Disability" means your
         inability, by reason of any physical or mental impairment, to
         substantially perform your duties and responsibilities hereunder for
         two or more periods of 90 days each in any 360-day period, as
         determined by a qualified physician with no history of prior dealings
         with you or the Company, as reasonably agreed upon by you (or, if you
         are unable to make such selection, by an adult member of your immediate
         family) and the Company. Such physician's written determination of your
         Permanent Disability

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         shall, upon delivery to the Company, be final and conclusive for
         purposes of this Agreement.

                  (c)      Your termination by the Company for "cause" as
         evidenced by, and effective upon, delivery by the Company to you of a
         Notice of Termination (as defined in Section 5 below). "Cause" shall
         mean, for purposes of this Agreement, (i) an act of fraud or
         embezzlement against the Company or an unauthorized disclosure of
         Confidential Information (as defined in Section (a)(iv) hereof) of the
         Company, in each case which is willful and results in material damage
         to the Company, (ii) any criminal violation of the Securities Act of
         1933 or the Securities Exchange Act of 1934, (iii) your conviction (or
         a plea of nolo contendere) of any felony, (iv) your gross neglect of
         your duties or your willful and continuing refusal to perform your
         duties, provided you have been given written notice of such neglect or
         refusal and within 30 days have failed to cure such neglect and
         refusal, or (v) your material willful misconduct with respect to the
         business or affairs of the Company.

                  (d)      Your termination of your employment for "good reason"
         by delivering to the Company a Notice of Termination (as defined in
         Section 5 below) not less than 30 days prior to the effective date of
         such termination. For purposes of this Agreement, "good reason" shall
         mean the occurrence of any of the events hereinafter set forth which
         are not cured by the Company within 30 days after the Company has
         received written notice from you specifying the particular events or
         conditions which constitute "good reason":

                           (i)      a material reduction in your duties, title,
                  responsibilities, authority, status, or reporting
                  responsibilities unless you have previously consented in
                  writing to such reduction (which consent may be given or
                  withheld in your sole discretion);

                           (ii)     a material reduction in your Base Salary or
                  the range of your target bonus; or;

                           (iii)    the Company's requiring you to be based more
                  than 35 miles from the Company's current headquarters in
                  Melville, New York or to any location for which the average
                  commute from your residence exceeds 45 minutes; or

                           (iv)     change of control (as defined in Section 7
                  hereof).

                  (e)      Termination of your employment by the Company
         "without cause" by delivery by the Company to you of a Notice of
         Termination (as defined in Section 5 below) not less than 30 days prior
         to the effective date of such termination. Your termination by the
         Company shall be considered to be "without cause" if you are terminated
         or dismissed by the Company for reasons other than death, permanent
         disability or for "cause".

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                  (f)      Your termination of your employment "without good
         reason" by delivery by you to the Company of a Notice of Termination
         (as defined in Section 5 below). Your termination of your employment
         shall be considered to be "without good reason" unless you resign for
         "good reason" (as defined in Section 4(d)).

         5.       Notice of Termination. Any termination by the Company or by
         you shall be communicated by a written "Notice of Termination" to the
         other party hereto. A "Notice of Termination" shall mean a notice which
         indicates a termination date and the specific termination provision in
         this Agreement relied upon and which sets forth in reasonable detail
         the facts and circumstances claimed to provide a basis for termination
         under the provision so indicated.

         6.       Payments Upon Termination.

                  (a)      Upon termination of your employment for any reason
         you will become entitled to (i) any accrued and unpaid Base Salary up
         to the date of termination, and (ii) any accrued and unpaid vacation
         pay up to the date of termination ((i) and (ii) being collectively
         referred to as the "Accrued Compensation").

                  (b)      Upon termination of your employment due to death or
         Permanent Disability, in addition to Accrued Compensation, you (or your
         estate, as the case may be) will become entitled to an amount equal to
         the bonus that you would have been entitled to receive for the fiscal
         year in which your termination occurs had you continued to be employed
         until the end of such fiscal year, multiplied by a fraction (i) the
         numerator of which is the number of days in such fiscal year through
         the termination date and (ii) the denominator or which is 365 (a
         "Pro-rata Bonus").

                  (c)      Upon a termination of your employment by the Company
         "without cause" or by you "for good reason" or upon a "Change of
         Control" (as defined in Section 7 hereof), in addition to Accrued
         Compensation, you will become entitled to (i) your Base Salary for 12
         months following the date of termination, (ii) your Pro-rata Bonus, and
         (iii) continued coverage for 12 months following termination under any
         health and dental program in which you were eligible to participate as
         of the time of termination of your employment.

                  (d)      You shall not be required to mitigate the amount of
         any payment provided for under this Section 6 by seeking other
         employment or otherwise and no payment shall be offset or reduced by
         the amount of any compensation or benefits provided to you in any
         subsequent employment. The Company's obligation to make the payments
         provided for in this Section 6 and otherwise perform its obligations
         hereunder shall not be affected by any circumstances, including,
         without limitation, set-off, counterclaim, recoupment, defense or other
         claim, right or action which the Company may have against you or
         others.

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         7.       Change of Control. For purposes of this Agreement, a "Change
         of Control" shall mean the approval by stockholders of the Company of
         (a) a merger or consolidation involving the Company if the stockholders
         of the Company, immediately before such merger or consolidation, do
         not, as a result of such merger or consolidation, directly or
         indirectly, continue to hold a majority of the voting power in the
         resulting entity, or (b) an agreement for the sale or other disposition
         of all or substantially all of the assets of the Company.

         8.       Prohibited Activities.

                  (a)      Certain Acknowledgements and Agreements.

                           (i)      We have discussed, and you recognize and
                  acknowledge the competitive and proprietary aspects of the
                  business of the Company.

                           (ii)     You acknowledge that your employment by the
                  Company creates a relationship of confidence and trust between
                  the Company and you with respect to certain information
                  relating to the business and affairs of the Company or
                  applicable to the business of any client, customer,
                  consultant, partner, external collaborator or service provider
                  of the Company, which may be made known to you by the Company
                  or by any client, customer, consultant, partner, external
                  collaborator or service provider of the Company, or learned by
                  you during the period of your affiliation with the Company.

                           (iii)    You further acknowledge that, while you are
                  employed hereunder, the Company will furnish, disclose or make
                  available to you Confidential Information (as defined in
                  Section 8 (a) (iv) below) related to the business of the
                  Company (whether or not the information has commercial value
                  to the Company's business). You also acknowledge that such
                  Confidential Information has been developed and will be
                  developed by the Company through the expenditure by the
                  Company of substantial time, effort and money and that all
                  such Confidential Information could be used by you to compete
                  with the Company. You also acknowledge that if you become
                  employed or affiliated with any competitor of the Company, it
                  is possible that you would disclose Confidential Information
                  to such competitor and would use Confidential Information,
                  knowingly or unknowingly, on behalf of such competitor.

                           (iv)     For purposes of this Agreement,
                  "Confidential Information" means confidential and proprietary
                  information of the Company, whether in written, oral,
                  electronic or other form, including, without limitation,
                  systems, processes, formulae, data, functional specifications,
                  computer software, programs and displays, know-how,
                  improvements, discoveries, inventions, developments, designs,
                  techniques, marketing plans, strategies, forecasts, new and
                  proposed products and technologies, unpublished financial
                  statements and financial information,

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                  business plans, budgets, projections, licenses, prices, costs,
                  training methods and materials, sales prospects, and customer,
                  supplier, manufacturer, collaborator, partner, and client
                  lists and any and all intellectual properties, including any
                  scientific, technical or trade secrets of the Company or of
                  any third party provided to you or the Company under a
                  condition of confidentiality, provided that Confidential
                  Information will not include information that is in the public
                  domain other than through any fault or act by you.

                  (b)      Covenants. While you are employed hereunder and for a
         period of one year following the termination of your employment
         hereunder for any reason or for no reason, you will not, without the
         prior written consent of the Company:

                           (i)      Engage, directly or indirectly, for your
                  benefit or the benefit of others, in any activity or
                  employment in the performance of which any Confidential
                  Information obtained during the course of your employment
                  would, by necessity, need to be disclosed by you in order to
                  engage in any such activity or employment. This covenant shall
                  not be construed to limit in any way your obligation not to
                  use or disclose Confidential Information as set forth in
                  Section 9 below.

                           (ii)     Either individually or on behalf of or
                  through any third party, directly or indirectly, solicit,
                  divert or appropriate or attempt to solicit, divert or
                  appropriate, any customers of the Company or any prospective
                  customers with respect to which the Company has developed or
                  made a sales presentation (or similar offering of services)
                  for the purpose of directly competing with the Company with
                  respect to the Company's "principal marketed products" (i.e.,
                  those products which are in the first or second detail
                  position) or its development candidates which have material
                  financial significance to the Company and which are in Phase
                  III programs; or

                           (iii)    Either individually or on behalf of or
                  through any third party, directly or indirectly, (A) solicit,
                  entice or persuade or attempt to solicit, entice or persuade
                  any employees of or consultants to the Company to leave the
                  service of the Company for any reason, or (B) employ, cause to
                  be employed, or solicit the employment of, any employees of or
                  consultants to the Company while any such person is providing
                  services to the Company or within six months after any such
                  person has ceased providing services to the Company; or

                           (iv)     Either individually or on behalf of or
                  through any third party, directly or indirectly, interfere
                  with, or attempt to interfere with, the relations between the
                  Company and any manufacturer or supplier to or customer of the
                  Company.

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                  (c)      Reasonableness of Restrictions. You understand that
         the provisions set forth in Section 8(b) are not meant to prevent you
         from earning a living or fostering your career. They are intended,
         however, to prevent competitors of the Company from gaining an unfair
         advantage from your knowledge of Confidential Information. You
         understand that, by making any other employer aware of the provisions
         set forth in this Section 8, that employer can take such action as to
         avoid your breach of this Section 8.

                  (d)      Survival of Acknowledgements and Agreements. Your
         acknowledgements and agreements set forth in this Section 8 will
         survive the termination of this Agreement and the termination of your
         employment hereunder for any reason or for no reason.

         9.       Protected Information. All Confidential Information shall be
         the sole property of the Company and its assigns. You hereby assign to
         the Company any right you may have or acquire in such Confidential
         Information. You will at all. times, both during the period while you
         are employed hereunder and after the termination of this Agreement and
         the termination of your employment hereunder for any reason or for no
         reason, maintain in confidence and will not, without the prior written
         consent of the Company, use, except as required in the course of
         performance of your duties for the Company or by court order, disclose
         or give to others any Confidential Information. In the event you are
         questioned by anyone not employed by the Company or by an employee of
         or a consultant to the Company not authorized to receive Confidential
         Information, in regard to any Confidential Information, or concerning
         any fact or circumstance relating thereto, you will promptly notify the
         Company. Upon the termination of your employment hereunder for any
         reason or for no reason, or if the Company otherwise requests, you will
         return to the Company all tangible Confidential Information and copies
         thereof (regardless how such Confidential Information or copies are
         maintained). The teems of this Section 9 are in addition to, and not in
         lieu of, any statutory or other contractual or legal obligation that
         you may have relating to the protection of the Company's Confidential
         Information. The terms of this Section 9 will survive indefinitely any
         termination of this Agreement and/or any termination of your employment
         hereunder for any reason or for no reason.

         10.      Ownership of Ideas, Copyrights and Patents.

                  (a)      Property of the Company. All ideas, discoveries,
         creations, manuscripts and properties, innovations, improvements,
         know-how, inventions, designs, developments, apparatus, techniques,
         methods, biological processes, cell lines, laboratory notebooks and
         formulae (collectively, the "Inventions") which may be used in the
         current or planned business of the Company or which in any way relates
         to such business, whether patentable, copyrightable or not, which you
         may conceive, reduce to practice or develop while you are employed
         hereunder (and, if based on or related to any Confidential Information,
         within two years after termination of such employment for any reason or
         for no reason), alone or in

                                       10
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         conjunction with another or others, whether during or out of regular
         business hours, whether or not on the Company's premises or with the
         use of its equipment, and whether at the request or upon the suggestion
         of the Company or otherwise, will be the sole and exclusive property of
         the Company, and that you will not publish any of the Inventions
         without the prior written consent of the Company. Without limiting the
         foregoing, you also acknowledge that all original works of authorship
         which are made by you (solely or jointly with others) within the scope
         of your employment or which relate to the business of the Company and
         which are protectable by copyright are "works made for hire" pursuant
         to the United States Copyright Act (17 U.S.C. Section 101). You will
         promptly disclose to the Company all of the foregoing and you hereby
         assign to the Company all of your right, title and interest in and to
         all of the foregoing. You further represent that, to the best of your
         knowledge and belief, none of the Inventions will violate or infringe
         upon any right, patent, copyright, trademark or right of privacy, or
         constitute libel or slander against or violate any other rights of any
         person, firm or corporation, and that you will use your best efforts to
         prevent any such violation.

                  (b)      Cooperation. At any time during your employment
         hereunder or after the termination of your employment hereunder for any
         reason or for no reason, you will cooperate fully with the Company and
         its attorneys and agents in the preparation and filing of all papers
         and other documents as may be required to perfect the Company's rights
         in and to any of such Inventions, including, without limitation,
         joining in any proceeding to obtain letters patent, copyrights,
         trademarks or other legal rights with respect to any such Inventions in
         the United States and in any and all other countries, provided that the
         Company will bear the expense of such proceedings, and that any patent
         or other legal right so issued to you personally will be assigned by
         you to the Company without charge by you.

                  (c)      Licensing and Use of Inventions. With respect to any
         Inventions, and work of any similar nature (from any source), whenever
         created, which you have not prepared or originated in the performance
         of your employment, but which you provide to the Company or incorporate
         in any Company product or system, you hereby grant to the Company a
         royalty-free, fully paid-up, non-exclusive, perpetual and irrevocable
         license throughout the world to use, modify, create derivative works
         from, disclose, publish, translate, reproduce, deliver, perform,
         dispose of, and to authorize others so to do, all such Inventions. You
         will not include in any Inventions you deliver to the Company or use on
         its behalf, without the prior written approval of the Company, any
         material which is or will be patented, copyrighted or trademarked by
         you or others unless you provide the Company with the written
         permission of the holder of any patent, copyright or trademark owner
         for the Company to use such material in a manner consistent with then
         current Company policy.

                  (d)      Prior Inventions. Listed on Exhibit 10(d) to this
         Agreement are any and all Inventions in which you claim or intend to
         claim any right, title and interest (collectively, "Prior Inventions"),
         including, without limitation, patent, copyright and trademark
         interests, which to the best of your knowledge will be or

                                       11
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         may be delivered to the Company in the course of your employment, or
         incorporated into any Company product or system. You acknowledge that
         your obligation to disclose such information is ongoing while you are
         employed hereunder.

         11.      Records. Upon termination of your employment hereunder for any
         reason or for no reason and at any other time requested by the Company,
         you will deliver to the Company any property of the Company which may
         be in your possession, including products, materials, memoranda, notes,
         records, reports, or other documents or photocopies of the same.

         12.      Representations. You hereby represent and warrant to the
         Company that you understand this Agreement, that you enter into this
         Agreement voluntarily and that your employment under this Agreement
         will not conflict with any legal duty owed by you to any other party,
         or with any agreement to which you are a party or by which you are
         bound, including, without limitation, any non-competition or
         non-solicitation provision contained in any such agreement.

         13.      General.

                  (a)      Notices. All notices, requests, consents and other
         communications hereunder which are required to be provided, or which
         the sender elects to provide, in writing, will be addressed to the
         receiving party's address set forth above or to such other address as a
         party may designate by notice hereunder, and will be either (i)
         delivered by hand, (ii) sent by overnight courier, or (iii) sent by
         registered or certified mail, return receipt requested, postage
         prepaid. All notices, requests, consents and other communications
         hereunder will be deemed to have been given either (i) if by hand, at
         the time of the delivery thereof to the receiving party at the address
         of such party set forth above, (ii) if sent by overnight courier, on
         the next business day following the day such notice is delivered to the
         courier service, or (iii) if sent by registered or certified mail, on
         the fifth business day following the day such mailing is made.

                  (b)      Entire Agreement. This Agreement, and the other
         agreements specifically referred to herein, embodies the entire
         agreement and understanding between the parties hereto with respect to
         the subject matter hereof and supersedes all prior oral or written
         agreements and understandings relating to the subject matter hereof. No
         statement, representation, warranty, covenant or agreement of any kind
         not expressly set forth in this Agreement will affect, or be used to
         interpret, change or restrict, the express terms and provisions of this
         Agreement.

                  (c)      Modifications and Amendments. The terms and
         provisions of this Agreement may be modified or amended only by written
         agreement executed by the parties hereto.

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<PAGE>

                  (d)      Waivers and Consents. The terms and provisions of
         this Agreement may be waived, or consent for the departure therefrom
         granted, only by written document executed by the party entitled to the
         benefits of such terms or provisions. No such waiver or consent will be
         deemed to be or will constitute a waiver or consent with respect to any
         other terms or provisions of this Agreement, whether or not similar.
         Each such waiver or consent will be effective only in the specific
         instance and for the purpose for which it was given, and will not
         constitute a continuing waiver or consent.

                  (e)      Assignment. The Company may assign its rights and
         obligations hereunder to any person or entity that succeeds to all or
         substantially all of the Company's business or that aspect of the
         Company's business in which you are principally involved or to any
         Company Affiliate; provided, that the Company shall remain responsible
         for any payments and obligations to you to the extent any assignee
         fails to fulfill such payments and obligations. You may not assign your
         rights and obligations under this Agreement without the prior written
         consent of the Company and any such attempted assignment by you without
         the prior written consent of the Company will be void.

                  (f)      Benefit. All statements, representations, warranties,
         covenants and agreements in this Agreement will be binding on the
         parties hereto and will inure to the benefit of the respective
         successors and permitted assigns of each party hereto. Nothing in this
         Agreement will be construed to create any rights or obligations except
         between the Company and you, except for your obligations to the Company
         as set forth herein, and no person or entity (except for a Company
         Affiliate as set forth herein) will be regarded as a third-party
         beneficiary of this Agreement.

                  (g)      Governing Law. This Agreement and the rights and
         obligations of the parties hereunder will be construed in accordance
         with and governed by the laws of the State of New York, without giving
         effect to the conflict of law principles thereof.

                  (h)      Jurisdiction, Venue and Service of Process. Any legal
         action or proceeding with respect to this Agreement that is not subject
         to arbitration pursuant to Section 14 (i) below will be brought in the
         courts of Suffolk County, New York. By execution and delivery of this
         Agreement, each of the parties hereto accepts for itself and in respect
         of its property, generally and unconditionally, the exclusive
         jurisdiction of the aforesaid courts.

                  (i)      Arbitration. Any controversy, dispute or claim
         arising out of or in connection with this Agreement, other than a
         controversy, dispute or claim arising under Section 8, 9 or 10 hereof,
         will be settled by final and binding arbitration to be conducted in New
         York, New York pursuant to the national rules for the resolution of
         employment disputes of the American Arbitration Association then in
         effect. The decision or award in any such arbitration will be final and
         binding upon the parties and judgment upon such decision or award may
         be entered in any

                                       13
<PAGE>

         court of competent jurisdiction or application may be made to any such
         court for judicial acceptance of such decision or award and an order of
         enforcement. In the event that any procedural matter is not covered by
         the aforesaid rules, the procedural law of New York will govern. Any
         disagreement as to whether a particular dispute is arbitrable under
         this Agreement shall itself be subject to arbitration in accordance
         with the procedures set forth herein. The fees of the arbitrators shall
         be paid by the Company.

                  (j)      WAIVER OF JURY TRIAL. ANY ACTION, DEMAND, CLAIM OR
         COUNTERCLAIM ARISING UNDER OR RELATING TO THIS AGREEMENT THAT IS NOT
         SUBJECT TO ARBITRATION PURSUANT TO SECTION 14(i) ABOVE WILL BE RESOLVED
         BY A JUDGE ALONE AND EACH OF YOU AND THE COMPANY WAIVE ANY RIGHT TO A
         JURY TRIAL THEREOF.

                  (k)      Severability. The parties intend this Agreement to be
         enforced as written. However, (i) if any portion or provision of this
         Agreement is to any extent declared illegal or unenforceable by a duly
         authorized court having jurisdiction, then the remainder of this
         Agreement, or the application of such portion or provision in
         circumstances other than those as to which it is so declared illegal or
         unenforceable, will not be affected thereby, and each portion and
         provision of this Agreement will be valid and enforceable to the
         fullest extent permitted by law and (ii) if any provision, or part
         thereof, is held to be unenforceable because of the duration of such
         provision, the geographic area covered thereby, or other aspect or
         scope of such provision, the court making such determination will have
         the power to reduce the duration, geographic area of such provision, or
         other aspect or scope of such provision, and/or to delete specific
         words and phrases ("blue-penciling"), and in its reduced or
         blue-penciled form, such provision will then be enforceable and will be
         enforced.

                  (l)      Injunctive Relief. You hereby expressly acknowledge
         that any breach or threatened breach of any of the terms and/or
         conditions set forth in Section 8, 9 or 10 of this Agreement will
         result in substantial, continuing and irreparable injury to the
         Company. Therefore, in addition to any other remedy that may be
         available to the Company, the Company will be entitled to injunctive or
         other equitable relief by a court of appropriate jurisdiction in the
         event of any breach or threatened breach of the terms of Section 8, 9
         or 10 of this Agreement. The period during which the covenants
         contained in Section 8 will apply will be extended by any periods
         during which you are found by a court to have been in violation of such
         covenants.

                  (m)      No Waiver of Rights, Powers and Remedies. No failure
         or delay by a party hereto in exercising any right, power or remedy
         under this Agreement, and no course of dealing between the parties
         hereto, will operate as a waiver of any such right, power or remedy of
         the party. No single or partial exercise of any right, power or remedy
         under this Agreement by a party hereto, nor any abandonment or
         discontinuance of steps to enforce any such right, power or

                                       14
<PAGE>

         remedy, will preclude such party from any other or further exercise
         thereof or the exercise of any other right, power or remedy hereunder.
         The election of any remedy by a party hereto will not constitute a
         waiver of the right of such party to pursue other available remedies.
         No notice to or demand on a party not expressly required under this
         Agreement will entitle the party receiving such notice or demand to any
         other or further notice or demand in similar or other circumstances or
         constitute a waiver of the rights of the party giving such notice or
         demand to any other or further action in any circumstances without such
         notice or demand.

                  (n)      Counterparts. This Agreement may be executed in two
         or more counterparts, and by different parties hereto on separate
         counterparts, each of which will be deemed an original, but all of
         which together will constitute one and the same instrument.

                  (o)      Opportunity to Review. You hereby acknowledge that
         you have had adequate opportunity to review these terms and conditions
         and to reflect upon and consider the terms and conditions of this
         Agreement, and that you have had the opportunity to consult with
         counsel of your own choosing regarding such terms. You further
         acknowledge that you fully understand the terms of this Agreement and
         have voluntarily executed this Agreement.

                  (p)      Survival of the Company's Obligations.
         Notwithstanding the termination of this agreement pursuant to Section
         4, the Company's obligation to make payments and provide benefits to
         you as set forth in Section 3 (g) (iv) and Section 6 will remain in
         effect.

                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]

                                       15
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         If the foregoing accurately sets forth our agreement, please so
indicate by signing and returning to us the enclosed copy of this Agreement.

                                             Very truly yours,

                                             OSI Pharmaceuticals, Inc.

                                             By:    /s/
                                                 -------------------------------
                                             Name: Colin Goddard, Ph.D
                                             Title: Chief Executive Officer

Accepted and Approved:

   /s/
---------------------------         -------------------
Gabe Leung                          Date

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                                  EXHIBIT 10(d)

                                PRIOR INVENTIONS