Exhibit 10.1

                                 PROMISSORY NOTE

$50,000.00                                                        March 26, 2003
                                                           San Diego, California

      1. Obligation. Plexus Vaccine Inc., a California corporation ("Plexus"),
hereby unconditionally promises to pay, in lawful money of the United States of
America, to the order of SIGA Technologies, Inc., a Delaware corporation
("SIGA"), the principal sum of $50,000, together with interest thereon at the
rate of six percent (6%) per annum, at the Maturity Date (defined below), at the
principal place of business of SIGA, New York City, New York. If Plexus fails to
pay any amounts due under this Note when due, interest shall accrue from the due
date until the date of payment at a rate of ten percent (10%) per annum, or to
the extent permitted by applicable law.

      2. Maturity Date. The principal and accrued interest due under this Note
shall become due and payable in full on the earliest of (i) the acquisition by
SIGA of substantially all of the assets of Plexus, (ii) three months following
the date when SIGA gives written notice to Plexus that SIGA does not intend to
proceed with the purchase of Plexus' assets, for reasons other than a default by
Plexus, (iii) ten days following a date when SIGA gives written notice to Plexus
that SIGA does not intend to proceed with the purchase of Plexus' assets, due to
a default by Plexus under the Plexus obligations set forth in the Asset Purchase
Agreement which the parties contemplate to be entered into within the next
several weeks, or (iv) November 30, 2003 (with the earliest of the foregoing
dates being the "Maturity Date" under this Note).

      3. Accelerated Maturity.

            (a) In the event Plexus makes an assignment for the benefit of
creditors, or appoints a receiver for all or substantially all of the Plexus
assets, or files a petition in bankruptcy or other similar proceedings for the
relief of debtors, or a creditor files a petition for bankruptcy of Plexus, then
upon the occurrence of any such event, the entire unpaid balance of principal
and accrued interest shall immediately become due and payable.

            (b) In the event Plexus undertakes any transaction involving the
consolidation or merger of Plexus with or into any individual, partnership,
limited liability company, venture, unincorporated association, organization,
syndicate, corporation, trust or trustee, executor, administrator or other legal
or personal representative or any government or any agency or political
subdivision thereof (a "Person") other than SIGA or the sale or conveyance to
any Person other than SIGA of all or substantially all of the assets of Plexus,
then upon the occurrence of any such event, the entire unpaid balance of
principal and accrued interest shall immediately become due and payable.

      4. Representations and Warranties. Plexus hereby represents and warrants
to SIGA:

            a. Enforceable Obligations. This Note has been duly executed and
delivered on behalf of Plexus, and constitutes the legal, valid and binding
obligation of Plexus, enforceable against it in accordance with its terms.

            b. No Legal Bar. The execution, delivery and performance of this
Note and the consummation of the transactions contemplated hereby, do not and
will not (i) violate any law, treaty, rule, regulation, right, privilege,
qualification or license or determination of an arbitrator or a court or other
governmental authority, in each case applicable to or binding upon Plexus or any
of its property or to which Plexus or any of its property is subject, or (ii)
conflict with or result in a breach of the terms or provisions of, or constitute
a default under, or result in the creation of any lien or other encumbrance
under any contractual obligation of the Plexus.


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      5. Cost of Collections. If this Note is not paid when due, whether at
maturity or by acceleration, Plexus promises to pay all reasonable costs
incurred by SIGA in collecting the amounts due and payable under this Note,
including reasonable attorney's fees and expenses.

      6. Waiver. Plexus hereby waives diligence, demand, presentment, protest
and notice of any kind, release, surrender or forbearance or other indulgence,
without notice.

      7. Modification. This Note may not be changed, modified, or terminated
orally, but only by an agreement in writing signed by the party to be charged.

      8. Governing Law. IN THE EVENT OF ANY LITIGATION WITH RESPECT TO THIS
NOTE, PLEXUS KNOWINGLY AND VOLUNTARILY WAIVES (TO THE EXTENT PERMITTED BY
APPLICABLE LAW) THE RIGHT TO A TRIAL BY JURY AND THE DEFENSES OF FORUM NON
CONVENIENS AND IMPROPER VENUE. THIS NOTE SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAWS OF THE STATE OF DELAWARE WITHOUT REGARD TO PRINCIPLES
OF CONFLICT OF LAWS AND SHALL BE BINDING UPON THE SUCCESSORS AND ASSIGNS OF
PLEXUS AND INURE TO THE BENEFIT OF SIGA AND ITS SUCCESSORS AND ASSIGNS. If any
item or provision of this Note shall be held invalid, illegal or unenforceable,
the validity of all other terms and provisions herein shall in no way be
affected thereby.

                                    PLEXUS VACCINE INC.


                                    By: /s/ Susan Burgess
                                        ----------------------------------------
                                    Name: Susan Burgess
                                    Title: President and Chief Executive Officer