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                                                                   EXHIBIT 10.91

                             CYTOTHERAPEUTICS, INC.
                          701 GEORGE WASHINGTON HIGHWAY
                           LINCOLN, RHODE ISLAND 02865

                                          As of July 1, 1999

John Schwartz
110 Atherton Avenue
Atherton, California 94027


Dear John:

      This letter will confirm our agreement with respect to the amendment, with
effect from July 1, 1999, of your Letter Agreement with CytoTherapeutics, Inc.
(the "Company"), dated December 19, 1999 (the "Agreement").

      Section 2. "Compensation; Time Commitment" shall be modified such that
subsection 2.a.(iii) shall read:

            "(iii) One Hundred Thirty Two Thousand Dollars ($132,000) per year,
      plus a fee of One Thousand Five Hundred Dollars ($1,500) per Board meeting
      or Committee meeting (if held at a date and time separate from the Board
      meeting) where you are physically present, plus Five Hundred Dollars
      ($500) per Board meeting or Committee meeting (if held at a date and time
      separate from the Board meeting) held by conference call, payable
      quarterly in arrears (this cash compensation plus any other compensation
      provided for herein shall be referred to as the "Compensation"). "

      Section 2. " Compensation; Time Commitment" shall be further modified such
that Subsection b. shall read:

            "b. As Chairman of the Board of Directors, you will be expected to
      devote no less than thirty (30) business days per calendar quarter to the
      performance of your duties and responsibilities collectively under this
      Agreement and the Consulting Agreement (hereinafter "Duties and
      Responsibilities"). In the event you devote more than thirty (30) days in
      any calendar quarter to the performance of your


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                                                                John J. Schwartz
                                                                    July 1, 1999
                                                                          Page 2


      Duties and Responsibilities, you shall, within thirty (30) days of the end
      of the calendar quarter, provide an accounting to the President and Chief
      Executive Officer of the Company detailing the actual time spent during
      such preceding calendar quarter. After review and approval by the
      President and Chief Executive Officer of the Company you will be promptly
      further compensated for additional days exceeding thirty (30) in any
      calendar quarter at the rate of One Thousand Five Hundred Dollars ($1,500)
      per day. All such additional payments made shall be promptly reported by
      the President and Chief Executive Officer to the Compensation Committee of
      the Board (the "Compensation Committee") for subsequent ratification by
      such Compensation Committee, such ratification not to delay the payment of
      any such additional payments."

Except as specifically modified hereby, all other terms and conditions of the
Agreement remain in full force and effect

If the foregoing is acceptable to you, please sign the enclosed copy of this
letter in the space provided below and return it to me, whereupon this letter
and such copy will constitute a binding amendment of the Agreement between you
and the Company on the basis set forth above as of the date first above written.

                                    Sincerely yours,
                                    CYTOTHERAPEUTICS, INC.


                                    By: __________________________________
                                          Richard M. Rose, M.D.
                                          President &
Accepted and Agreed:                      Chief Executive Officer


_______________________________
John J. Schwartz

Date: ___________________


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