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                                                                 EXHIBIT 10.4.28


                                    ADDENDUM

THIS ADDENDUM is executed with respect to the Promissory Note in the original
principal amount of $ 1,000,000.00 dated September 25, 1998 [and maturing on
August 31, 1999] (the "Note"), made by Neoprobe Corporation (referred to in this
Addendum as "Borrower") payable to the order of Bank One, NA ("Lender"), and is
hereby incorporated into and made a part of the Note. Terms used in this
Addendum with their initial letters capitalized are used as defined in the Note,
unless they are otherwise expressly defined herein.

1.   The paragraph in the Note having the caption "VARIABLE INTEREST RATE" is
     hereby deleted in its entirety and the following is hereby substituted in
     lieu thereof:

     VARIABLE INTEREST RATE. Subject to designation of a different interest rate
     index by Borrower as provided below, the interest rate on this Note is
     subject to fluctuation based upon the Prime Rate of interest in effect from
     time to time (the "Index") (which rate may not be the lowest, best or most
     favorable rate of interest which Lender may charge on loans to its
     customers). "Prime Rate" shall mean the rate announced from time to time by
     Lender as its prime rate. Each change in the variable interest rate to be
     charged on this Note will become effective without notice on the same day
     as the Index changes. Except as otherwise provided herein, the unpaid
     principal balance of this Note will accrue interest at a rate per annum
     which will from time to time be equal to the sum of the Index, plus 0.00%.

     INTEREST RATE OPTIONS. The following interest rate options are available
     under this Note:

     (a) Default Option. The interest rate margin and index described in the
     "VARIABLE INTEREST RATE" paragraph above (the "Default Option").

     (b) ONE MONTH LONDON INTERBANK OFFERED RATE. A margin of 2.25 percentage
     points over the one month LONDON INTERBANK OFFERED RATE ("LIBOR"). For
     purposes of this Note, the one month LIBOR rate shall mean the one month
     offered rate for U.S. Dollar deposits of not less than $1,000,000.00 as of
     11:00 A.M. City of London, England time two London Business Days prior to
     the first date of each one month Interest Period (as defined below) of this
     Note as shown on the display designated as "British Bankers Assoc. Interest
     Settlement Rates" on the Dow Jones Telerate Service ("Telerate"), Page 3750
     (or such other page as may replace that page on that service for the
     purpose of displaying such rate). Provided, however, that if such LIBOR
     rate is not available on Telerate then such offered rate shall be otherwise
     independently determined by Lender from an alternate, substantially similar
     independent source or service available to Lender or shall be calculated by
     Lender by a substantially similar methodology as that theretofore used to
     determine such offered rate in Telerate.

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     (c) TWO MONTH LONDON INTERBANK OFFERED RATE. A margin of 2.25 percentage
     points over the two month LONDON INTERBANK OFFERED RATE ("LIBOR"). For
     purposes of this Note, the two month LIBOR rate shall mean the two month
     offered rate for U.S. Dollar deposits of not less than $1,000,000.00 as of
     11:00 A.M. City of London, England time two London Business Days prior to
     the first date of each two month Interest Period (as defined below) of this
     Note as shown on the display designated as "British Bankers Assoc. Interest
     Settlement Rates" on the Dow Jones Telerate Service ("Telerate"), Page 3750
     (or such other page as may replace that page on that service for the
     purpose of displaying such rate). Provided, however, that if such LIBOR
     rate is not available on Telerate then such offered rate shall be otherwise
     independently determined by Lender from an alternate, substantially similar
     independent source or service available to Lender or shall be calculated by
     Lender by a substantially similar methodology as that theretofore used to
     determine such offered rate in Telerate

     (d) THREE MONTH LONDON INTERBANK OFFERED RATE. A margin of 2.25 percentage
     points over the three month LONDON INTERBANK OFFERED RATE ("LIBOR"). For
     purposes of this Note, the three month LIBOR rate shall mean the three
     month offered rate for U.S. Dollar deposits of not less than $1,000,000.00
     as of 11:00 A.M. City of London, England time two London Business Days
     prior to the first date of each three month Interest Period (as defined
     below) of this Note as shown on the display designated as "British Bankers
     Assoc. Interest Settlement Rates" on the Dow Jones Telerate Service
     ("Telerate"), Page 3750 (or such other page as may replace that page on
     that service for the purpose of displaying such rate). Provided, however,
     that if such LIBOR rate is not available on Telerate then such offered rate
     shall be otherwise independently determined by Lender from an alternate,
     substantially similar independent source or service available to Lender or
     shall be calculated by Lender by a substantially similar methodology as
     that theretofore used to determine such offered rate in Telerate.

     (e) SIX MONTH LONDON INTERBANK OFFERED RATE. A margin of 2.25 percentage
     points over the six month LONDON INTERBANK OFFERED RATE ("LIBOR"). For
     purposes of this Note, the six month LIBOR rate shall mean the six month
     offered rate for U.S. Dollar deposits of not less than $1,000,000.00 as of
     11:00 A.M. City of London, England time two London Business Days prior to
     the first date of each six month Interest Period (as defined below) of this
     Note as shown on the display designated as "British Bankers Assoc. Interest
     Settlement Rates" on the Dow Jones Telerate Service ("Telerate"), Page 3750
     (or such other page as may replace that page on that service for the
     purpose of displaying such rate). Provided, however, that if such LIBOR
     rate is not available on Telerate then such offered rate shall be otherwise
     independently determined by Lender from an alternate, substantially similar
     independent source or service available to Lender or shall be calculated by
     Lender by a substantially similar methodology as that theretofore used to
     determine such offered rate in Telerate.

     If used in this Note, "London Business Day" means any day other than a
     Saturday, Sunday or a day on which banking institutions are generally
     authorized or obligated by law or executive order to close in the City of
     London, England.

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     When the interest rate is based on a LIBOR rate, a Treasury Securities Rate
     or a Fixed Rate (an "Option Rate"), the rate shall be in effect for a
     period of the number of months as indicated in the rate option description
     (the "Interest Period"), in any case extended to the next succeeding
     business day when necessary, beginning on a borrowing date, conversion date
     or expiration date of the then current Interest Period. Adjustments in the
     interest rate due to changes in the maximum nonusurious interest rate
     allowed (the "Highest Lawful Rate") shall be made on the effective day of
     any change in the Highest Lawful Rate.

     Provided Borrower is not in default under this Note, Borrower may designate
     in advance which of the above interest rate indexes shall be applicable to
     any loan advance under this Note. In the absence of any such designation
     the interest rate option shall be the Default Option. Thereafter unpaid
     principal balances under this Note may be converted (at the end of an
     Interest Period if the index used to determine the interest rate therefore
     is an Option Rate) to another of the above interest rate options, or
     continued for an additional interest period, when applicable, as designated
     by Borrower in advance; and in the absence of sufficient advance
     designation as determined by the Lender as to conversion to or continuation
     of an Option Rate index, the rate shall be converted to the Default Option.
     Notwithstanding the foregoing, an Option Rate index may not be elected for
     a loan or advance under this Note, nor any conversion to or continuation of
     an Option Rate index be elected, if the Interest Period thereof would
     extend beyond the maturity of this Note.

     Each loan or advance under this Note at conversion into or continuation of
     an Option Rate shall be in a minimum amount of $250,000.00. Unless
     otherwise provided herein, accrued interest on amounts for which the
     interest rate is based on a LIBOR rate or a Treasury Securities Rate shall
     be due and payable at the end of the respective Interest Period therefor.

     Borrower shall pay to Lender from time to time such amounts as Lender may
     determine to be necessary to compensate Lender for any costs incurred by
     Lender which Lender determines are attributable to its making or
     maintaining any LIBOR rates hereunder or its obligation to make any such
     LIBOR rates hereunder, or any reduction in ant amount receivable by Lender
     under this Note in respect of any such rates or such obligation (such
     increases in costs and reductions in amounts receivable being herein called
     "Additional Costs"), resulting from any change after the date of this Note
     in U.S. federal, state, municipal, or foreign laws or regulations
     (including Regulation D), or the adoption or making after such date of ant
     interpretations, directives, or requirements applying to a class of banks
     including Lender of or under any U.S. federal, state, municipal, or any
     foreign laws or regulations (whether or not having the force of law) by any
     court or governmental or monetary authority charged with the interpretation
     or administration thereof ("Regulatory Change"), which: (1) changes the
     basis of taxation of any amounts payable to Lender under this Note in
     respect of any such LIBOR rates (other than taxes imposed on the overall
     net income of the Lender); or (2) imopses or modifies any reserve, special
     deposit, compulsory loan, or similar requirements relating to any
     extensions of credit or other assets of, or any deposits with or other
     liabilities of. Lender (including any of such

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     LIBOR rates or any deposits referred to in the definition of any LIBOR
     rate); or (3) imposes any other condition affecting this Note (or any of
     such extensions of credit or liabilities). Lender will notify the Borrower
     of any event occurring after the date of this Note which will entitle
     Lender to compensation pursuant to this paragraph as promptly as
     practicable after it obtains knowledge thereof and determines to request
     such compensation. Determinations by Lender for purposes of this paragraph
     of the effect of any Regulatory change in its costs of making or
     maintaining LIBOR rates or on amounts receivable by it in respect of LIBOR
     rates, and of the additional amounts required to compensate Lender in
     respect of any Additional Costs, shall be presumes prima facie correct.

     In respect of any LIBOR Loans, in the event that Lender shall have
     determined that dollar deposits of the relevant amount for the relevant
     Interest Period for such LIBOR Loans are not available or that, by reason
     of circumstances affecting such market, adequate and reasonable means do
     not exist for ascertaining the LIBOR rate applicable to such Interest
     Period, as the case may be, Lender shall promptly give notice of such
     determination to the Borrower and (i) any notice of new LIBOR Loans (or
     conversion of existing loans to LIBOR Loans) previously given by the
     Borrower and not yet borrowed (or converted, as the case may be) shall be
     deemed a notice to make loans bearing interest at the Default Option, and
     (ii) the Borrower shall be obligated either to prepay or to convert any
     outstanding LIBOR Loans on the last day of the then current Interest Period
     or Periods with respect thereto, as Borrower shall elect.

     Without prejudice to any other provisions of this Note, the Borrower agrees
     to indemnify Lender against any loss or expense which Lender may sustain or
     incur as a consequence of any default by the Borrower in payment when due
     of any amount due hereunder in respect of any LIBOR Loan, including, but
     not limited to, any loss of profit, premium or penalty incurred by Lender
     in respect of funds borrowed by it for the purpose of making or maintaining
     such LIBOR Loan, as determined by Lender in the exercise of its sole but
     reasonable discretion. A certificate as to any such loss or expense shall
     be promptly submitted by Lender to the Borrower and shall, in the absence
     of manifest error, be conclusive and binding as to the amount thereof.

     If at any time any new law, treaty or regulation enacted after the date
     hereof, or any change after the date hereof in any existing law, treaty or
     regulation, or any interpretation thereof after the date hereof by any
     governmental or other regulatory authority charged with the administration
     thereof, shall make it unlawful for Lender to fund any LIBOR Loans which it
     is committed to make hereunder with moneys obtained in the Eurodollar
     market, the commitment of Lender to fund LIBOR Loans shall, upon the
     happening of such event forthwith be suspended for the duration of such
     illegality, and Lender shall by written notice to the Borrower declare that
     the commitment with respect to such loans has been so suspended and, if and
     when such illegality ceases to exist, such suspension shall cease and
     Lender shall similarly notify the Borrower. If any such change shall make
     it unlawful for Lender to continue in effect the funding in the applicable
     Eurodollar market of any LIBOR Loan previously made by it hereunder, Lender
     shall, upon the happening of such event, notify the Borrower in writing
     stating the reasons therefor, and the Borrower shall, on the earlier of (i)
     the last day of the then current Interest Period or (ii) if required by
     such law, regulation or

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     interpretation, on such date as shall be specified in such notice, either
     convert all LIBOR Loans to loans bearing interest at the Default Option or
     prepay all LIBOR Loans to Lender in full, as Borrower shall elect.

     Lender may, but shall not be required to, make LIBOR Loans hereunder with
     funds obtained outside the United States.

2.   The paragraph of the Note having the caption "PREPAYMENT" is hereby deleted
     in its entirety and the following paragraph is hereby substituted in lieu
     thereof:

     PREPAYMENT. Borrower may prepay all or any portion of the principal balance
     outstanding on this Note which is accruing at the Default Option at any
     time without payment of premium or penalty. Borrower may not prepay all or
     any portion of the principal balance outstanding on this Note which is
     accruing interest based on a LIBOR rate without obtaining Lender's prior
     written consent, which Lender can grant or refuse to grant in its sole
     discretion. All permitted prepayments shall be applied to the indebtedness
     owing hereunder in such order and manner as Lender may from time to time
     determine.

3.   The paragraph of the Note having the caption "LENDER'S RIGHTS" is hereby
     deleted in its entirety and the following paragraph is hereby substituted
     in lieu thereof:

     LENDER'S RIGHTS. Upon default, Lender may, at its option, without further
     notice or demand (i) declare the entire unpaid principal balance on this
     Note, all accrued unpaid interest and all other coats and expenses for
     which Borrower is responsible under this Note and any other Related
     Document immediately due, (ii) refuse to advance any additional amounts
     under this Note, (iii) foreclose all liens securing payment hereof, (iv)
     pursue any other rights, remedies and recourses available to Lender,
     including without limitation, any such rights, remedies or recourses under
     the Related Documents, at law or in equity, or (v) pursue any combination
     of the foregoing. Upon default, including failure to pay upon final
     maturity, Lender, at its option, may also, if permitted by applicable law,
     do one or both of the following: (a) accrue interest on this Note at the
     Default Option and increase the variable interest rate on this Note to 3.00
     percentage points over the Default Option, and (b) add any unpaid accrued
     interest to principal and such sum will bear interest therefrom until paid
     at the rate provided in this Note (including any increased rate). The
     interest rate will not exceed the maximum rate permitted by applicable law.
     This Note has been delivered to Lender and is performable in Ohio. Courts
     within the State of Ohio have jurisdiction over any dispute arising under
     or pertaining to this Note and venue for such dispute shall be in Franklin
     County, Ohio. THIS NOTE SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE
     WITH THE LAWS OF THE STATE OF OHIO AND APPLICABLE FEDERAL LAWS.

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4.   Except as expressly modified by this Addendum, all of the terms and
     conditions of the Note continue unchanged and in full force and effect.

5.   The paragraph of the Note having the caption "CONFESSION OF JUDGMENT" is
     hereby ratified and confirmed.

Dated with effect as of the date of the Note.

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WARNING - BY SIGNING THIS PAPER YOU GIVE UP YOUR RIGHT TO NOTICE AND COURT
TRIAL. IF YOU DO NOT PAY ON TIME A COURT JUDGMENT MAY BE TAKEN AGAINST YOU
WITHOUT YOUR PRIOR KNOWLEDGE AND THE POWERS OF A COURT CAN BE USED TO COLLECT
FROM YOU REGARDLESS OF ANY CLAIMS YOU MAY HAVE AGAINST THE CREDITOR WHETHER FOR
RETURNED GOODS, FAULTY GOODS, FAILURE ON HIS PART TO COMPLY WITH THE AGREEMENT,
OR ANY OTHER CAUSE.
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Neoprobe Corporation

By:   /s/ Brent L. Larson
     -----------------------------
Its:  Vice President, Finance