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                                                                   EXHIBIT 4.4.a
 
                                    March 31, 1997

Connecticut Development Authority
845 Brook Street
Rocky Hill, CT 06067

         Re:      Modification and Conversion of Note and Related Matters

Gentlemen:

          This letter will evidence the agreement of Bio-Plexus, Inc. ("BP") and
the Connecticut Development Authority ("CDA") regarding the modification and
conversion of a certain note held by CDA and related matters.

          On March 7, 1995 CDA and BP entered into a financing transaction
pursuant to which CDA advanced BP $2.5 million and BP issued CDA a note in that
principal amount (the "Note). The principal amount of the Note has been paid
down to $2,121,410 as of the date of this letter agreement.

          BP desires to repay a portion of the Note and CDA desires to convert
the unpaid balance of the Note into shares of BP common stock, without par value
("Shares").

          In consideration of the mutual and dependent promises set forth
herein, BP and CDA agree as follows:

          1.       Modification of Note

                   The Note is hereby amended to provide for it to be
          convertible into Shares in accordance with this letter agreement.

          2.       Repayment of the Note

                   Upon entering into this letter agreement, BP will pay
          $1,050,000 against the principal amount of the Note and CDA will
          release all liens against the assets of BP related to the Note.

          3.       Conversion of Balance of Note

                  Upon entering into this letter agreement, CDA will convert the
         remaining balance of the Note, including interest through March
         31,1997, ($1,087,320.81) into 241,627 Shares at $4.50 per Share. The
         Note will be surrendered to BP by CDA and BP will issue CDA
         certificates for such Shares.

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          4.       Sale of Shares

                   Subsequent to the conversion of the Note CDA may hold or sell
          the CDA Shares. Any sale of more than an average of 1,000 Shares per
          trading day will require the consent of BP, which consent will not be
          unreasonably withheld. The sole concern of BP with respect to any such
          sale is to minimize the adverse impact of such a sale upon the public
          market for the Shares. However, in no event will CDA be required by
          this paragraph to hold any Shares beyond March 31, 1998.

          5.       Redemption of CDA Shares

                   BP will redeem Shares issued hereunder and held by CDA upon
          the following terms and conditions:

                           i.       If CDA has received net aggregate proceeds
                                    from the sale of Shares equal to $1,125,000,
                                    BP will redeem all remaining Shares issued
                                    to CDA hereunder at a price of one cent per
                                    Share. CDA shall notify BP if such event
                                    occurs and BP shall promptly redeem the
                                    remaining Shares.

                           ii.      If by March 31, 1998, CDA has not received
                                    net aggregate proceeds of $1,125,000 from
                                    the sale of Shares issued hereunder and CDA
                                    still holds some Shares, BP will redeem all
                                    such remaining Shares for an aggregate
                                    redemption price equal to the difference
                                    between $1,125,000 and the amount received
                                    by CDA for the sale of all Shares issued
                                    hereunder other than said remaining Shares.

                           iii.     Redemption payments shall be made in cash.

         6.       Representations and Warrants of BP

                  BP represents and warrants to CDA as follows:

                  a.       BP has all requisite corporate power and authority to
                           enter into this Letter Agreement and to issue and
                           sell the Shares. The issuance and sale of the shares
                           by BP to CDA has been duly and validly authorized by
                           all requisite corporate proceeding on the part of BP
                           and do not constitute or will not result in: (i) a
                           conflict, default or violation of or under the
                           Certificate of Incorporation or By-laws of BP; (ii) a
                           default or violation under any mortgage, note, lease
                           agreement or other instrument or obligation to which
                           BP is a party; or (iii) a violation of any statute,
                           rule, regulation, order, judgment or decree of any
                           court, public body or authority with jurisdiction
                           over BP; or (iv)
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                           creation of imposition of any lien, charge or
                           encumbrance of any property of BP; or (v) an event
                           which would require any consent from any third party
                           which has not been obtained.

                  b.       Upon the issuance and delivery of the shares to CDA,
                           all of such Shares shall have been duly authorized
                           and validly issued and fully paid and nonassessable,
                           subject to no grant of rights, liens, or other
                           rights, claims or options.

                  c.       BP has the requisite corporate power and authority to
                           execute, deliver and perform its obligations under
                           this letter agreement. The execution and delivery of
                           this agreement and BP's performance of its
                           obligations hereunder have been duly and validly
                           authorized by BP and this agreement, as executed and
                           delivered, will constitute a valid and binding
                           obligation of BP enforceable against BP in accordance
                           with its terms.

                  d.       Immediately prior to the issuance of the Shares,
                           7,517,869 shares of BP are authorized, issued and
                           outstanding. The average daily trading volume of the
                           Shares for each full week of March, 1997 was 30,840,
                           29,880, 58,660, and 5,940, respectively; and for each
                           such week the average trading price per Share was
                           $5.95, $5.55, $5.03 and $5.30.

                  e.       The Shares are currently listed and traded on the
                           NASDAQ Small Capital Market (the "Exchange"). BP is a
                           reporting company under the Securities and Exchange
                           Act of 1934 (the "Act"), and is current in the filing
                           of all information and reports required thereunder.
                           BP has no plans to discontinue such listing or
                           trading.

                  f.       The business of BP has been and is presently being
                           conducted in compliance with applicable federal,
                           state and local governmental laws, rules, regulations
                           and ordinances. There are no suits, actions, claims,
                           investigations or proceedings pending, or, to the
                           knowledge of BP, threatened against BP and there are
                           no outstanding orders, judgments, injunctions, awards
                           or decrees pending against BP, which alone or in
                           combination could have a material adverse effect upon
                           BP.

                  g.       There is no existing, condition or event which at
                           present or which with the passage of time can
                           reasonably be expected to have a material adverse
                           change in the business, assets, operations, affairs,
                           prospects or financial condition of BP.

                  h.       BP has all franchise, permits, licenses and other
                           similar authorizations necessary for the conduct of
                           its business as now being conducted and 
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                           contemplated and is not aware of any state of facts
                           that would make impossible or impractical to continue
                           or obtain any similar authorizations necessary for
                           the conduct of its business as planned to be
                           conducted.

         7.       Conditions of Closing

                  a.       As part of the conversion of the Note, CDA will
                           receive an opinion of counsel to BP substantially to
                           the effect that:

                           i.       The Shares issued to CDA have been duly
                                    authorized, validly issued and are fully
                                    paid and nonassessable, free and clear of
                                    any adverse liens, claims or encumbrances.

                           ii.      The Shares may be freely traded pursuant to
                                    Rule 144 promulgated by the Securities and
                                    Exchange Commission.

                           iii.     No authorizations, approvals, or permits of
                                    any governmental authority or regulatory
                                    body, is required in connection with the
                                    lawful issuance and sale of the Shares to
                                    CDA.

                  b.       BP will pay for all of the reasonable fees, costs and
                           expenses of CDA incurred in connection with this
                           transaction, including, without limitation, the
                           reasonable fees and costs of CDA's counsel.

         8.       Additional Agreements

                  a.       Subject to the terms hereof, this letter agreement
                           constitutes the entire agreement of the parties as to
                           the repayment of the Note and supersedes all prior
                           agreements whether written or oral related to the
                           Note other than the warrants for Shares issued in
                           connection with the Note and the terms of the Letter
                           Agreement dated June 14, 1995 as such terms relate to
                           such warrants. Specifically, but not in limitation of
                           the foregoing, the Loan Agreement and Security
                           Agreement each dated March 7, 1995 and the Collateral
                           Assignment and Security Agreement, dated July 27,
                           1993 as amended on March 7, 1995 are hereby cancelled
                           and are of no further force or effect. Warrants for
                           Shares issued in connection with the Note and the
                           terms of the Letter Agreement dated June 14, 1995
                           remain in full force and effect except as modified
                           herein.

                  b.       CDA and BP will take such further actions as are
                           necessary or desirable to effectuate the purposes of
                           the foregoing terms and conditions.
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                  c.       This letter agreement may not be modified except by a
                           writing executed by both parties.

                  d.       This agreement shall be binding upon and inure to the
                           benefit of the parties and their respective
                           successors and assigns.

                  e.       This agreement shall be governed by and construed
                           under the laws of the State of Connecticut.

         9.       Effective Date

                  The parties agree that the effective date of this letter
         agreement for all purposes shall be March 31, 997.

         10.      Notices

                  Any notice or other communication of one party to the other
         shall be in writing and delivered by facsimile transaction, hand
         delivery or U.S. mail addressed to CDA at the above address and
         addressed to BP at 384 Q Merrow Road, Tolland, CT 06084. Any notice
         shall be effective upon receipt.

         If the foregoing accurately reflects CDA's understanding of its
agreement with BP on the matters set forth above, please sign and return a copy
of this letter agreement to BP.

                                    Sincerely,

                                    BIO-PLEXUS, INC.

                                    By:_______________________________

Accepted and Agreed:

CONNECTICUT DEVELOPMENT AUTHORITY

By:_____________________________