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                                                                   EXHIBIT 10.50

                     FIRST AMENDMENT TO EMPLOYMENT AGREEMENT

         This AMENDMENT TO EMPLOYMENT AGREEMENT (the "First Amendment") made as
of July 1, 2001 between ARIAD Pharmaceuticals, Inc., a Delaware corporation (the
"Company"), and Timothy P. Clackson, Ph.D. (the "Employee").

         The Company and the Employee have entered into an Employment Agreement
dated as of June 8, 2000 (the "Agreement"), and the parties hereto desire to
further amend certain provisions of the Agreement.

         NOW, THEREFORE, in consideration of the premises set forth herein and
for other good and valuable consideration, the receipt of which is hereby
acknowledged, the parties hereto agree to further amend the Agreement as
follows:

         I. Term of Employment. The first sentence of Section 2 is hereby
amended to read as follows:

                  "The term of the Employee's employment under the Agreement is
         hereby extended to December 31, 2003 (the "Term"), unless sooner
         terminated pursuant to Section 4 or 5 of this Agreement; provided,
         however, that this Agreement shall automatically be renewed for
         successive one-year terms (the Term and, if the period of employment is
         so renewed, such additional period(s) of employment are collectively
         referred to herein as the "Term") unless terminated by written notice
         given by either party to the other at least 90 days prior to the end of
         the applicable Term."

         II. Compensation. Section 3.1 is hereby replaced and amended in its
entirety as follows:

                  "3.1. As full compensation for all services to be rendered
         pursuant to this Agreement, the Company agrees to pay the Employee a
         salary at the fixed rate of $185,000 per annum as of the date hereof
         and increased thereafter during the Term, by amounts, if any, to be
         determined by the Board of Directors of the Company (the "Board") in
         its sole discretion, payable in equal semi-monthly installments, less
         such deductions or amounts to be withheld as shall be required by
         applicable law and regulations."

         III. Definitions. The definition of the Company's "Field of Interest"
in Section 14 (b) of the Agreement is hereby amended to read as follows:

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                  The "Company's Field of Interest" is the discovery,
         development and commercialization of (i) pharmaceutical products based
         on (a) intervention in signal transduction pathways and (b) gene and
         cell therapy, and (ii) technologies to regulate genes and proteins. The
         Company's Field of Interest may be changed at any time at the sole
         discretion of the Company.

         IV. This Amendment shall be governed by and construed and enforced in
accordance with the laws of the Commonwealth of Massachusetts applicable to
agreements made and to be performed entirely in Massachusetts.

         V. Except as modified by this First Amendment, the Agreement remains in
full force and effect and unchanged.

                  IN WITNESS WHEREOF, the parties have executed this First
Amendment as of the date first written above.


                                ARIAD PHARMACEUTICALS, INC.


                                By:      /s/ Harvey J. Berger
                                    -----------------------------------
                                    Harvey J. Berger, M.D.
                                    Chairman and Chief Executive Officer


                                EMPLOYEE

                                   /s/ Timothy P. Clackson
                                --------------------------------
                                Timothy P. Clackson, Ph.D.

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