SECURED PROMISSORY NOTE



$36,000.00                                                    September 24, 1998


        FOR VALUE RECEIVED, Owen A. Dempsey, an individual with an address at 21
Harris Street, Brookline, Massachusetts 02146 (the "Maker"), promises to pay to
the order of Endogen, Inc. ("Endogen") the principal sum of THIRTY-SIX THOUSAND
AND 00/100 DOLLARS ($36,000.00), together with interest thereon from the date
hereof on the unpaid principal balance from time to time outstanding, on the
earliest to occur of: (a) one year from the date hereof, (b) the date the Maker
ceases employment or other business relationship with Endogen or (c) the sale or
transfer by the Maker of any of his shares of Endogen's Common Stock (any event
described in (a), (b) or (c) above shall be referred to hereinafter as a
"Payment Due Date"). Interest on the unpaid principal balance hereof shall
accrue from and including the date hereof to the date such principal amount is
paid at the annual rate of 5.42% compounded annually. Interest on the unpaid
principal balance of this Secured Promissory Note shall be payable on the
Payment Due Date, and in the event of failure of payment on the Payment Due
Date, monthly thereafter until this Secured Promissory Note is paid in full.

        Payments of both principal and interest are to be made at Endogen's
principal offices located at 30 Commerce Way, Woburn, Massachusetts 01801 or
such other place as Endogen designates to the Maker in writing, in lawful money
of the United States of America. Interest shall be computed on the basis of a
360-day year and twelve 30-day months.

        No delay or omission on the part of Endogen in exercising any right
hereunder shall operate as a waiver of such right or of any other right of
Endogen, nor shall any delay, omission or waiver on any one occasion be deemed a
bar to or waiver of the same or any other right on any future occasion.

        This Secured Promissory Note may be prepaid at any time, in whole or in
part, by the Maker without penalty.

        This Secured Promissory Note is secured by and entitled to the benefits
of a pledge of securities pursuant to a Pledge Agreement, dated the date hereof,
by the Maker in favor of Endogen (the "Pledge Agreement"). Upon the occurrence
of an Event of Default (as defined in the Pledge Agreement), Endogen may declare
any or all obligations or liabilities of the Maker to Endogen (including the
unpaid principal hereunder and any interest due thereof), immediately due and
payable without presentment, demand, protest or notice. The Maker shall remain
personally liable to Endogen with respect to all of the obligations under this
Secured Promissory Note.


<PAGE>

        When this Secured Promissory Note has been paid in full, the pledge of
the collateral pursuant to the Pledge Agreement shall cease, and the collateral
pursuant to the Pledge Agreement shall revert to the Pledgor free and clear of
all liens securing any obligation or liability of the Pledgor to Endogen, and
Endogen's rights, title, and interest therein shall cease and become void.

        The Maker hereby waives presentment, demand, protest, and notice of
every kind. The Maker shall pay on demand all costs, including court costs and
reasonable attorney's fees, paid or incurred by Endogen in enforcing this
Secured Promissory Note upon default.

        This Secured Promissory Note shall be governed by and construed in
accordance with the laws of the Commonwealth of Massachusetts.

        IN WITNESS WHEREOF, the Maker has caused this Secured Promissory Note to
be signed under seal, as of the date first above written.


                                                       MAKER



Witness:  /s/Avery W. Catlin                           /s/Owen A. Dempsey
          ----------------------                       -------------------------
                                                       Owen A. Dempsey