<PAGE>

                                                                  EXHIBIT 10.31
                                                                  -------------

                                PROMISSORY NOTE
                                ---------------

$206,000.00                                                      March 13, 2000

     FOR VALUE RECEIVED, the undersigned, Daniel R. Omstead ("Maker"), hereby
promises to pay to the order of Reprogenesis, Inc., a Texas corporation
("Payee"), the principal sum of $206,000 upon the earlier of receipt of any
payment by Maker pursuant to the Daniel Omstead Severance Agreement dated
February 14, 2000 between Maker and Payee or December 31, 2000, without
interest.

     Maker may from time to time prepay all or any portion of the outstanding
balance of this note on any Business Day without penalty or premium.  As used
herein, a "Business Day" shall mean any day other than a Saturday, Sunday or
other day on which banks in Texas are permitted or required by law to be closed.
All payments and prepayments made in accordance with this note in respect of
principal on this note are to be made in lawful money of the United States of
America no later than 12 o'clock noon, central time, in same day funds, at 21
Erie Street, Suite 22, Cambridge, Massachusetts 02139, or such other place as
the holder hereof shall designate in writing to Maker.  Maker shall have no
right to reborrow under this note any amounts paid or prepaid in respect of
principal on this note.

     If any payment or prepayment of principal on this note shall become due on
a day which is not a Business Day, such payment shall be made on the next
succeeding Business Day.

     In addition to all principal on this note, Maker agrees to pay (a) all
reasonable costs and expenses incurred by all owners and holders of this note in
collecting this note through any probate, reorganization, bankruptcy or any
other proceeding and (b) reasonable attorneys' fees when and if this note is
placed in the hands of an attorney for collection after demand.

     Maker and any and all endorsers, guarantors and sureties severally waive
grace, presentment for payment, notice of dishonor, notice of intent to
accelerate, notice of acceleration, protest and notice of protest and diligence
in collecting and bringing of suit against any party hereto, and agree to all
modifications, renewals, extensions, substitutions or replacements hereof or
partial payments hereon and to any release or substitution of security, if any,
hereof, in whole or in part, with or without notice, before or after maturity.
This note may be transferred by Payee, and the rights and privileges of Payee
under this note shall inure to the benefit of Payee's representatives,
successors and assigns.

     Maker hereby expressly waives presentment, demand, protest or other notice
of any kind to Maker.
<PAGE>

     THIS NOTE SHALL BE DEEMED TO BE A CONTRACT UNDER THE LAWS OF THE STATE OF
TEXAS AND FOR ALL PURPOSES SHALL BE CONSTRUED AND ENFORCED IN ACCORDANCE WITH
THE LAWS OF SAID STATE AND APPLICABLE FEDERAL LAWS.  Maker further agrees that
the consideration furnished to Maker is related solely to business, commercial,
investment or other similar purposes and not personal, family or agricultural
purposes.

     THIS WRITTEN AGREEMENT REPRESENTS THE FINAL AGREEMENT BETWEEN THE PARTIES
AND MAY NOT BE CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS, OR SUBSEQUENT
ORAL AGREEMENTS OF THE PARTIES.  THERE ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN
THE PARTIES.


                                             -------------------------
                                             Daniel R. Omstead