<PAGE>


                                                                   EXHIBIT 10.21



                FIFTH AMENDMENT TO CREDIT AND SECURITY AGREEMENT

                  This Amendment, dated as of November 13, 2001, is made by and
between LECTEC CORPORATION, a Minnesota corporation (the "Borrower"), and WELLS
FARGO BUSINESS CREDIT, INC., a Minnesota corporation (the "Lender").

                                    RECITALS

                  The Borrower and the Lender have entered into a Credit and
Security Agreement dated as of November 22, 1999 as amended by that certain
First Amendment to Credit and Security Agreement dated as of February 9, 2000,
that certain Second Amendment to Credit and Security Agreement dated as of
September 26, 2000, that certain Third Amendment to Credit and Security
Agreement dated as of September 28, 2000, that certain Fourth Amendment to
Credit and Security Agreement dated as of October 26, 2000, a Waiver Agreement
dated November 16, 2000 and a Waiver Agreement dated March 23, 2001 (as so
amended and modified, the "Credit Agreement"). Capitalized terms used in these
recitals have the meanings given to them in the Credit Agreement unless
otherwise specified.

                  The Borrower has requested that certain amendments be made to
the Credit Agreement, which the Lender is willing to make pursuant to the terms
and conditions set forth herein.

                  NOW, THEREFORE, in consideration of the premises and of the
mutual covenants and agreements herein contained, it is agreed as follows:

                  1. Defined Terms. Capitalized terms used in this Amendment
which are defined in the Credit Agreement shall have the same meanings as
defined therein, unless otherwise defined herein. In addition, Section 1.1 of
the Credit Agreement is amended by adding or amending, as the case may be, the
following definitions:

                  "Collateral" means all of the Borrower's Accounts, chattel
         paper, deposit accounts, documents, Equipment, General Intangibles,
         goods, instruments, Inventory, Investment Property, letter-of-credit
         rights, letters of credit, all sums on deposit in any Collateral
         Account, and any items in any Lockbox; together with (i) all
         substitutions and replacements for and products of any of the
         foregoing; (ii) in the case of all goods, all accessions; (iii) all
         accessories, attachments, parts, equipment and repairs now or hereafter
         attached or affixed to or used in connection with any goods; (iv) all
         warehouse receipts, bills of lading and other documents of title now or
         hereafter




<PAGE>

         covering such goods; (v) all collateral subject to the Lien of any
         Security Document; (vi) any money, or other assets of the Borrower that
         now or hereafter come into the possession, custody, or control of the
         Lender; (vii) all sums on deposit in the Special Account; and (viii)
         proceeds of any and all of the foregoing.

                  "Documentary L/C Amount" means the sum of (i) the aggregate
         face amount of any issued and outstanding documentary Letters of Credit
         and (ii) the unpaid amount of the Obligation of Reimbursement with
         respect to documentary Letters of Credit.

                  "L/C Amount" means the sum of the Documentary L/C Amount and
         the Standby L/C Amount.

                  "Maximum Line" means $2,000,000 unless said amount is reduced
         pursuant to Section 2.10, in which event it means the amount to which
         said amount is reduced.

                  "Note" or "Revolving Note" means the Borrower's revolving
         promissory note, payable to the order of the Lender in substantially
         the form of Exhibit A hereto, as the same may hereafter be amended,
         supplemented or restated from time to time, and any note or notes
         issued in substitution therefor, as the same may hereafter be amended,
         supplemented or restated from time to time and any note or notes issued
         in substitution therefor.

         In addition, the definitions of and all references to "Eximbank
Adjusted Maximum Line," "Eximbank Agreement," "Eximbank Note," and "Tangible Net
Worth" are hereby deleted.

                  2. Rules of Interpretation. Section 1.2 of the Credit
Agreement is amended to read as follows:

                  "Section 1.2 Other Definitional Terms; Rules of
         Interpretation. The words "hereof", "herein" and "hereunder" and words
         of similar import when used in this Agreement shall refer to this
         Agreement as a whole and not to any particular provision of this
         Agreement. All accounting terms not otherwise defined herein have the
         meanings assigned to them in accordance with GAAP. All terms defined in
         the UCC and not otherwise defined herein have the meanings assigned to
         them in the UCC. References to Articles, Sections, subsections,
         Exhibits, Schedules and the like, are to Articles, Sections and
         subsections of, or Exhibits or Schedules attached to, this Agreement
         unless otherwise expressly provided. The words "include", "includes"
         and "including" shall be deemed to be followed by the phrase "without
         limitation". Unless the context in which used herein otherwise clearly
         requires, "or" has the inclusive meaning represented by the phrase
         "and/or". Defined terms include in the singular number the plural and
         in the plural number the singular. Reference to any agreement
         (including the Loan Documents), document or instrument means such
         agreement, document or instrument as amended or modified and in effect
         from time to time in accordance with the terms thereof (and, if
         applicable, in accordance with the terms hereof and the other Loan
         Documents), except where otherwise explicitly provided,





<PAGE>

         and reference to any promissory note includes any promissory note which
         is an extension or renewal thereof or a substitute or replacement
         therefor. Reference to any law, rule, regulation, order, decree,
         requirement, policy, guideline, directive or interpretation means as
         amended, modified, codified, replaced or reenacted, in whole or in
         part, and in effect on the determination date, including rules and
         regulations promulgated thereunder."

                  3.     Letters of Credit. Section 2.2 of the Credit Agreement
is hereby deleted in its entirety and replaced with the following:

         Section 2.2 Letters of Credit.

                  (a)    The Lender agrees, on the terms and subject to the
                         conditions herein set forth, to cause an Issuer to
                         issue, from the Funding Date to the Termination Date,
                         one or more irrevocable standby or documentary letters
                         of credit (each, a "Letter of Credit") for the
                         Borrower's account. The Lender shall have no obligation
                         to cause an Issuer to issue any Letter of Credit if the
                         face amount of the Letter of Credit to be issued would
                         exceed the lessor of:

                  (b)    (i) with respect to irrevocable standby Letters of
                         Credit, $250,000 less the Standby L/C Amount, plus

                         (ii) with respect to documentary Letters of Credit,
                         $300,000 less the Documentary L/C Amount; or

                         (iii)  with respect to the aggregate of irrevocable
                         standby Letters of Credit plus documentary Letters of
                         Credit, the Borrowing Base less the sum of (A) all
                         outstanding and unpaid Revolving Advances and (B) the
                         L/C Amount.

                  (c)    Each Letter of Credit, if any, shall be issued pursuant
                         to a separate L/C Application entered into by the
                         Borrower and the Lender for the benefit of the Issuer,
                         completed in a manner satisfactory to the Lender and
                         the Issuer. The terms and conditions set forth in each
                         such L/C Application shall supplement the terms and
                         conditions hereof, but if the terms of any such L/C
                         Application and the terms of this Agreement are
                         inconsistent, the terms hereof shall control.

                  (d)    No Letter of Credit shall be issued with an expiry date
                         later than the Termination Date in effect as of the
                         date of issuance.

                  (e)    Any request to cause an Issuer to issue a Letter of
                         Credit under this Section 2.2 shall be deemed to be a
                         representation by the Borrower that the conditions set
                         forth in Section 4.2 have been satisfied as of the date
                         of the request.


<PAGE>

                  4. Minimum Interest. Section 2.6(b) of the Credit Agreement is
hereby deleted in its entirety and replaced with the following:

                  Section 2.6 (b) MINIMUM INTEREST CHARGE. Notwithstanding the
         interest payable pursuant to Section 2.6(a) and 2.6(c), (i) so long as
         all activity hereunder relates to Letters of Credit and there are no
         Advances hereunder, the Borrower shall pay to the Lender interest of
         not less than $50,000 per year of this Agreement or (ii) if there are
         any Advances hereunder, the Borrower shall pay to the Lender Interest
         of not less than $100,000 per year of this Agreement (the "Minimum
         Interest Charge") during the term of this Agreement, and the Borrower
         shall pay any deficiency between the Minimum Interest Charge and the
         amount of interest otherwise calculated under Sections 2.6(a) and
         2.6(c) on the date and in the manner provided in Section 2.8.

                  5. Unused Line Fee. The Lender agrees that so long as all
activity under the Credit Agreement relates to Letters of Credit and there are
no Advances, the unused line fee required under Section 2.7(b) of the Credit
Agreement shall be waived.

                  6. Letter of Credit Fees. Section 2.7(c) of the Credit
Agreement is hereby deleted in its entirety and replaced with the following:

                  Section 2.7(c) LETTER OF CREDIT FEES.

                     (i) Irrevocable Standby Letters of Credit. The
                     Borrower agrees to pay the Lender a fee with respect
                     to each irrevocable standby Letter of Credits, if any,
                     accruing on a daily basis and computed at the annual
                     rate of two percent (2%) of the aggregate amount that
                     may then be drawn on all issued and outstanding
                     irrevocable standby Letters of Credit assuming
                     compliance with all conditions for drawing thereunder
                     (the "Aggregate Standby Face Amount"), from and
                     including the date of issuance of such irrevocable
                     standby Letter of Credit until such date as such
                     irrevocable standby Letter of Credit shall terminate
                     by its terms or be returned to the Lender, due and
                     payable monthly in arrears on the first day of each
                     month and on the Termination Date; provided, however
                     that during Default Periods, in the Lender's sole
                     discretion and without waiving any of its other rights
                     and remedies, such fee shall increase to four percent
                     (4%) of the Aggregate Standby Face Amount. The
                     foregoing fee shall be in addition to any and all
                     fees, commissions and charges of any Issuer of a
                     Letter of Credit with respect to or in connection with
                     such irrevocable standby Letter of Credit.

                     (ii) Documentary Letters of Credit. The Borrower
                     agrees to pay the Lender a fee with respect to each
                     documentary Letter of Credit, if any, computed at two
                     percent (2%) of the face amount of such documentary
                     Letter of Credit, due and payable upon issuance of
                     such documentary Letter of Credit; provided, however
                     that during Default Periods, in the

<PAGE>

                     Lender's sole discretion and without waiving any of its
                     other rights and remedies, such fee shall increase to
                     four percent (4%). The foregoing fee shall be in
                     addition to any and all fees, commissions and charges
                     of any Issuer of a Letter of Credit with respect to or
                     in connection with such Letter of Credit.

                  7. Maturity Date. The next "Maturity Date" under Section 2.10
shall be November 22, 2003.

                  8. Financing Statements. Section 3.6 of the Credit Agreement
is amended by adding the following new sentence before the first sentence of
that Section:

         "The Borrower authorizes the Lender to file from time to time where
         permitted by law, such financing statements against collateral
         described as "all personal property" as the Lender deems necessary or
         useful to perfect the Security Interest."

                  9. Minimum Book Net Worth. Section 6.12 of the Credit
Agreement is hereby deleted its entirety and replaced with the following:

                  Section 6.12 Minimum Book Net Worth. The Borrower will
         maintain, during each period described below, its Book Net Worth,
         determined as at the end of each month, at an amount not less than the
         amount set forth opposite such period:

<Table>
<Caption>

                    Period Ended                Minimum Book Net Worth
                    ------------                ----------------------
              <S>                              <C>
                  October 31, 2001                    $4,607,000

                 November 30, 2001                    $4,264,000

                 December 31, 2001                    $3,769,000

                  January 31, 2002                    $3,577,000

                 February 28, 2002                    $3,429,000

                   March 31, 2002                     $3,226,000

                   April 30, 2002                     $3,098,000

                    May 31, 2002                      $2,874,000

                   June 30, 2002                      $2,933,000
</Table>

                  10. Minimum Earnings/Maximum Loss Before Taxes. Section 6.13
of the Credit Agreement is hereby deleted in its entirety and replaced with the
following:

                  Section 6.13 Minimum Earnings/Maximum Loss Before Taxes. The
         Borrower will achieve during each period described below, year-to-date
         Earnings or Loss




<PAGE>

         Before Taxes, of not less than/greater than the amount set forth
         opposite such period [bracketed amounts indicate maximum Loss Before
         Taxes]:

<Table>
<Caption>

                                                        Minimum
                                                        -------
                                                 Earnings/Maximum Loss
                                                 ---------------------
                        Period Ended                  Before Taxes
                        ------------                  ------------
                    <S>                          <C>
                     October 31, 2001                 [$1,479,000]

                     November 30, 2001                [$1,822,000]

                     December 31, 2001                [$2,318,000]

                     January 31, 2002                  [$ 292,000]

                     February 28, 2002                 [$ 440,000]

                      March 31, 2002                   [$ 643,000]

                      April 30, 2002                   [$ 771,000]

                       May 31, 2002                    [$ 995,000]

                       June 30, 2002                   [$ 936,000]
</Table>



                  11. New Covenants. Section 6.14 of the Credit Agreement is
hereby deleted in its entirety and replaced with the following:

                  Section 6.14 New Covenants. On or before June 30, 2002, the
         Borrower and the Lender shall agree on new financial covenant levels
         for periods after such date. The new covenant levels will be based on
         the Borrower's projections for such periods and shall be no less
         stringent than the present levels.

                  12. Tangible Net Worth Covenant. Section 6.15 of the Credit
Agreement relating to a Tangible Net Worth Covenant is hereby deleted in its
entirety.

                  13. Capital Expenditures. Section 7.10 of the Credit Agreement
is hereby deleted in its entirety and replaced with the following:

                  Section 7.10 Capital Expenditures. The Borrower will not incur
         or contract to incur Capital Expenditures of more than $150,000 in any
         single transaction or more than $350,000 in the aggregate during any
         fiscal year.

                  14. No Other Changes. Except as explicitly amended by this
Amendment, all of the terms and conditions of the Credit Agreement shall remain
in full force and effect and shall apply to any advance or letter of credit
thereunder.



<PAGE>


                  15. Conditions Precedent. This Amendment shall be effective
when the Lender shall have received an executed original hereof, together with
each of the following, each in substance and form acceptable to the Lender in
its sole discretion:

                  (a)      The Note, executed by the Borrower;

                  (b) A Certificate of the Secretary of the Borrower certifying
         as to (i) the resolutions of the board of directors of the Borrower
         approving the execution and delivery of this Amendment, (ii) the fact
         that the articles of incorporation and bylaws of the Borrower, which
         were certified and delivered to the Lender pursuant to the Certificate
         of Authority of the Borrower's secretary or assistant secretary dated
         as of November 22, 1999, continue in full force and effect and have not
         been amended or otherwise modified except as set forth in the
         Certificate to be delivered, and (iii) certifying that the officers and
         agents of the Borrower who have been certified to the Lender, pursuant
         to the Certificate of Authority of the Borrower's secretary or
         assistant secretary dated as of November 22, 1999, as being authorized
         to sign and to act on behalf of the Borrower continue to be so
         authorized or setting forth the sample signatures of each of the
         officers and agents of the Borrower authorized to execute and deliver
         this Amendment and all other documents, agreements and certificates on
         behalf of the Borrower.

                  (c)      Such other matters as the Lender may require.

                  16. Representations and Warranties. The Borrower hereby
represents and warrants to the Lender as follows:

                  (a) The Borrower has all requisite power and authority to
         execute this Amendment and to perform all of its obligations hereunder,
         and this Amendment has been duly executed and delivered by the Borrower
         and constitutes the legal, valid and binding obligation of the
         Borrower, enforceable in accordance with its terms.

                  (b) The execution, delivery and performance by the Borrower of
         this Amendment have been duly authorized by all necessary corporate
         action and do not (i) require any authorization, consent or approval by
         any governmental department, commission, board, bureau, agency or
         instrumentality, domestic or foreign, (ii) violate any provision of any
         law, rule or regulation or of any order, writ, injunction or decree
         presently in effect, having applicability to the Borrower, or the
         articles of incorporation or by-laws of the Borrower, or (iii) result
         in a breach of or constitute a default under any indenture or loan or
         credit agreement or any other agreement, lease or instrument to which
         the Borrower is a party or by which it or its properties may be bound
         or affected.

                  (c) All of the representations and warranties contained in
         Article V of the Credit Agreement are correct on and as of the date
         hereof as though made on and as of




<PAGE>

         such date, except to the extent that such representations and
         warranties relate solely to an earlier date.

                  17. References. All references in the Credit Agreement to
"this Agreement" shall be deemed to refer to the Credit Agreement as amended
hereby; and any and all references in the Security Documents to the Credit
Agreement shall be deemed to refer to the Credit Agreement as amended hereby.

                  18. No Waiver. The execution of this Amendment and acceptance
of any documents related hereto shall not be deemed to be a waiver of any
Default or Event of Default under the Credit Agreement or breach, default or
event of default under any Security Document or other document held by the
Lender, whether or not known to the Lender and whether or not existing on the
date of this Amendment.

                  19. Release. The Borrower hereby absolutely and
unconditionally releases and forever discharges the Lender, and any and all
participants, parent corporations, subsidiary corporations, affiliated
corporations, insurers, indemnitors, successors and assigns thereof, together
with all of the present and former directors, officers, agents and employees of
any of the foregoing, from any and all claims, demands or causes of action of
any kind, nature or description, whether arising in law or equity or upon
contract or tort or under any state or federal law or otherwise, which the
Borrower has had, now has or has made claim to have against any such person for
or by reason of any act, omission, matter, cause or thing whatsoever arising
from the beginning of time to and including the date of this Amendment, whether
such claims, demands and causes of action are matured or unmatured or known or
unknown.

                  20. Costs and Expenses. The Borrower hereby reaffirms its
agreement under the Credit Agreement to pay or reimburse the Lender on demand
for all costs and expenses incurred by the Lender in connection with the Loan
Documents, including without limitation all reasonable fees and disbursements of
legal counsel. Without limiting the generality of the foregoing, the Borrower
specifically agrees to pay all fees and disbursements of counsel to the Lender
for the services performed by such counsel in connection with the preparation of
this Amendment and the documents and instruments incidental hereto. The Borrower
hereby agrees that the Lender may, at any time or from time to time in its sole
discretion and without further authorization by the Borrower, make a loan to the
Borrower under the Credit Agreement, or apply the proceeds of any loan, for the
purpose of paying any such fees, disbursements, costs and expenses.

                  21. Miscellaneous. This Amendment may be executed in any
number of counterparts, each of which when so executed and delivered shall be
deemed an original and all of which counterparts, taken together, shall
constitute one and the same instrument.


<PAGE>


                  IN WITNESS WHEREOF, the parties hereto have caused this
Amendment to be duly executed as of the date first written above.



WELLS FARGO BUSINESS CREDIT, INC.        LECTEC CORPORATION



By   /s/Terry Jackson                    By   /s/Douglas Nesbit
   ---------------------------------        ---------------------------------
    Terry S. Jackson                          Douglas Nesbit
    Its Vice President                        Its Chief Financial Officer






<PAGE>


                                     Exhibit A to Credit and Security Agreement

                                 REVOLVING NOTE

$2,000,000.00                                            Minneapolis, Minnesota
                                                              November 13, 2001

                  For value received, the undersigned, LECTEC CORPORATION, a
Minnesota corporation (the "Borrower"), hereby promises to pay on the
Termination Date under the Credit Agreement (defined below), to the order of
WELLS FARGO BUSINESS CREDIT, INC., a Minnesota corporation (the "Lender"), at
its main office in Minneapolis, Minnesota, or at any other place designated at
any time by the holder hereof, in lawful money of the United States of America
and in immediately available funds, the principal sum of Two Million and 00/100
Dollars ($2,000,000.00) or, if less, the aggregate unpaid principal amount of
all Revolving Advances made by the Lender to the Borrower under the Credit
Agreement (defined below) together with interest on the principal amount
hereunder remaining unpaid from time to time, computed on the basis of the
actual number of days elapsed and a 360-day year, from the date hereof until
this Note is fully paid at the rate from time to time in effect under the Credit
and Security Agreement dated November 22, 1999 (as amended, supplemented or
restated from time to time, the "Credit Agreement") by and between the Lender
and the Borrower. The principal hereof and interest accruing thereon shall be
due and payable as provided in the Credit Agreement. This Note may be prepaid
only in accordance with the Credit Agreement.

                  This Note is issued pursuant, and is subject, to the Credit
Agreement, which provides, among other things, for acceleration hereof. This
Note is the Revolving Note referred to in the Credit Agreement. This Note is
secured, among other things, pursuant to the Credit Agreement and the Security
Documents as therein defined, and may now or hereafter be secured by one or more
other security agreements, mortgages, deeds of trust, assignments or other
instruments or agreements.

                  The Borrower hereby agrees to pay all costs of collection,
including attorneys' fees and legal expenses in the event this Note is not paid
when due, whether or not legal proceedings are commenced.

                  Presentment or other demand for payment, notice of dishonor
and protest are expressly waived.

                                      LECTEC CORPORATION


                                      By   /s/Douglas Nesbit
                                         -------------------------------------
                                           Douglas Nesbit
                                           Its Chief Financial Officer