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                 SUPPLEMENTAL AGREEMENT TO AGREEMENT CONCERNING
            CONVERSION OF CONVERTIBLE NOTE AND CONNECTICUT PRESENCE

         SUPPLEMENTAL AGREEMENT, dated as of November 23, 1999, between
Genaissance Pharmaceuticals, Inc. (the "Company") and Connecticut Innovations,
Incorporated (the "Investor").

         WHEREAS, the Company and the Investor are parties to a certain
Agreement Concerning Conversion of Convertible Note and Connecticut Presence
(the "Existing Agreement") (capitalized terms used and not otherwise defined
herein shall have the meanings assigned to such terms in the Existing
Agreement); and

         WHEREAS, as of the date hereof, the Company has issued a certain
Mandatory Convertible Promissory Note (the "Note") to the Investor in the amount
of $500,000; and

         WHEREAS, as of the date hereof, the Company has issued a certain Common
Stock Purchase Warrant (the "Warrant") to the Investor; and

         WHEREAS, the Note is convertible, upon the terms set forth therein,
into certain shares (the "Preferred Shares") of the Series B Preferred Stock of
the Company; and

         WHEREAS, the Warrant is exercisable, upon the terms set forth therein,
for certain shares (the "Warrant Shares") of the Common Stock of the Company;
and

         WHEREAS, the parties hereto desire to provide the Investor with certain
put and other rights with respect to the Preferred Shares and the Warrant Shares
substantially similar to the put and other rights that are set forth in Section
5 of the Existing Agreement.

         NOW THEREFORE, in consideration of the foregoing, and for good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the parties hereto, intending to be legally bound, do hereby agree
as follows:

         1. The Preferred Shares and the Warrant Shares shall be treated as if
they are Preferred Shares and Common Shares, respectively, under the Existing
Agreement and, as such, shall be subject to the Put provisions of Section 5 of
the Existing Agreement; PROVIDED, HOWEVER, that

         (a)      Section 5(c)(x)(ii), as it applies to the Preferred Shares,
 shall read as follows:

                  an amount equal to the Series B Preferred Stock Issue Price
                  (as defined in the Note) of such Share plus an amount
                  calculated to yield to the Investor an aggregate twenty-five
                  percent (25%) annually compounded rate of return on such share
                  of Series B Preferred Stock for the period from the
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                  Series B Preferred Stock Issue Date (as defined in the
                  Note) through and including the date when the Put Price is
                  paid to the Investor, less the amount of any dividends
                  previously paid to the Investor with respect to such Share.

         (b)      Section 5(c)(y)(ii), as it applies to the Warrant Shares,
shall read as follows:

                  an amount equal to the Adjusted Exercise Price of such Share
                  (at the time of exercise) plus an amount calculated to yield
                  to the Investor an aggregate twenty-five percent (25%)
                  annually compounded rate of return on the Adjusted Exercise
                  Price of such Share for the period from the date of exercise
                  through and including the date when the Put Price is paid to
                  the Investor, less the amount of any dividends previously paid
                  to the Investor with respect to such Share.

         (c)      for purposes of Section 5(c), the Put Price as to each share
of Common Stock underlying each unexercised Warrant shall be the excess, if any,
of the Current Market Price (as defined in the Existing Agreement) of one share
of Common Stock purchasable upon exercise of such Warrant over the Adjusted
Exercise Price (as defined and determined under the Warrant) of such Warrant as
of the date of determination.

         2.       The terms and conditions of this Agreement shall inure to the
benefit of and be binding upon the respective successors and assigns of the
parties (including transferees of the Preferred Shares and the Warrant Shares);
PROVIDED, HOWEVER, that the provisions of Section 1 hereof shall not succeed to
or be assigned to any non-affiliate of the Investor.

         3.       Notices given under this Agreement shall be given in the same
fashion as under the Existing Agreement.

         4.       This Agreement shall be governed by and construed under the
laws of the State of Connecticut, without regard to principles of conflicts of
laws and rules of such state.

         5.       This Agreement may be executed in two or more counterparts
(including by way of facsimile), each of which shall be deemed an original, but
all of which together shall constitute one and the same instrument.

                      [THE NEXT PAGE IS THE SIGNATURE PAGE]
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         IN WITNESS WHEREOF, the undersigned have hereunto set their hands as of
the day and year first above written.

                                GENAISSANCE PHARMACEUTICALS, INC.



                                By:  /s/ Kevin Rakin
                                     ----------------------------------
                                     Name: Kevin Rakin
                                     Title:   Executive Vice President

                                CONNECTICUT INNOVATIONS, INCORPORATED



                                By:  /s/ Victor R. Budnick
                                     ----------------------------------
                                     Name:  Victor R. Budnick
                                     Title:  Executive Director