Revolving Promissory Note

$850,000                                                  Woburn, Massachusetts
                                                                August 28, 1996

        FOR VALUE RECEIVED, the undersigned, ENDOGEN, INC., a Massachusetts
corporation (the "Borrower"), promises to pay to the order of Silicon Valley
Bank, a California-chartered bank ("Bank"), at such place as the holder hereof
may designate, in lawful money of the United States of America, the aggregate
unpaid principal amount of all advances ("Advances") made by Bank to Borrower in
accordance with the terms of the Loan and Security Agreement between Borrower
and Bank of even date herewith, as amended from time to time (the "Loan
Agreement"), up to a maximum principal amount of EIGHT HUNDRED FIFTY THOUSAND
AND NO/100THS Dollars ($850,000), until paid in full. Borrower shall also pay
interest on the aggregate unpaid principal amount of such Advances at the rates
and in accordance with the terms of the Loan Agreement. The entire principal
amount and all accrued interest shall be due and payable on AUGUST 27, 1997.

        Borrower irrevocably waives the right to direct the application of any
and all payments at any time hereafter received by Bank from or on behalf of
Borrower, and Borrower irrevocably agrees that Bank shall have the continuing
exclusive right to apply any and all such payments against the then due and
owing obligations of Borrower as Bank may deem advisable. In the absence of a
specific determination by Bank with respect thereto, all payments shall be
applied in the following order: (a) then due and payable fees and expenses; (b)
then due and payable interest payments and mandatory prepayments; and (c) then
due and payable principal payments and optional prepayments.

        Bank is hereby authorized by Borrower to endorse on Bank's books and
records each Advance made by Bank under this Note and the amount of each payment
or prepayment of principal of each such Advance received by Bank; it being
understood, however, that failure to make any such endorsement (or any error in
notation) shall not affect the obligations of Borrower with respect to Advances
made hereunder, and payments of principal by Borrower shall be credited to
Borrower notwithstanding the failure to make a notation (or any errors in
notation) thereof on such books and records.

        Borrower promises to pay Bank all costs and expenses of collection of
this Note and to pay all reasonable attorneys' fees incurred in such collection,
whether or not there is a suit or action, or in any suit or action to collect
this Note or in any appeal thereof. Borrower waives presentment, demand,
protest, notice of protest, notice of dishonor, notice of nonpayment, and any
and all other notices and demands in connection with the delivery, acceptance,
performance, default or enforcement of this Note, as well as any applicable
statutes of limitations. No delay by Bank in exercising any power or right
hereunder shall operate as a waiver of any power or right. Time is of the
essence as to all obligations hereunder.

        This Note is issued pursuant to the Loan Agreement, which shall govern
the rights and obligations of Borrower with respect to all obligations
hereunder.

        This Note shall be deemed to be made under, and shall be construed in
accordance with and governed by, the laws of the Commonwealth of Massachusetts,
excluding conflicts of laws principles.

        Executed as an instrument under seal.

                                               ENDOGEN, INC.

                                               By: /s/Owen A. Dempsey
                                               -------------------------------
                                                   Owen A. Dempsey, President

ATTEST /s/Dan Burns
- --------------------
       Dan Burns, Clerk