<PAGE>   1
THIS EXHIBIT HAS BEEN REDACTED AND IS THE SUBJECT OF A CONFIDENTIAL TREATMENT
REQUEST. REDACTED MATERIAL IS MARKED WITH "*" AND BRACKETS AND HAS BEEN FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION.

                                                                EXHIBIT 10.2(b)

                     AMENDMENT TO THE RESELLER AGREEMENT
                       DATED 22ND DAY OF JULY, 1996 AND
                            24TH DAY OF JULY, 1997


         The Reseller Agreement between Tropix, Inc. ("Tropix") and Digene
Corporation ("Digene") is hereby amended as follows:


Schedule A is amended as follows:


                                  SCHEDULE-A


                        PRODUCT AND MINIMUM PURCHASES


Product:          [*******] CDP-Star(R) with [************] of
                  Emerald-II(TM)Enhancer in [**************].

Minimum Annual Purchases For Product Purchased for Use in Human In Vitro
Diagnostics:

                  [**] Liters of Product in [**********************]
                         [***] Liters of Product in [******]
                         [***] Liters of Product in [******]
                         [***] Liters of Product in [******]

Minimum Annual Purchases For Product Purchased for Pharmaceutical Screening:


                           [**] Liters of Product in [*****************]
                         [***] Liters of Product in each Fiscal Year thereafter



*FY = Fiscal Year is July 1 through June 30



<PAGE>   2

THIS EXHIBIT HAS BEEN REDACTED AND IS THE SUBJECT OF A CONFIDENTIAL TREATMENT
REQUEST. REDACTED MATERIAL IS MARKED WITH "*" AND BRACKETS AND HAS BEEN FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION.

Schedule B is amended as follows:

                                  SCHEDULE B

                             PRICES AND ROYALTIES

PRICES

Price per liter of Product based upon aggregate purchases per calendar year:

[******]       [******]          [*****************]
[*****]        [******]          [*****]                $[***]/liter
[********]     [*********]       [*********]            $[***]/liter
[*****]        [*****]           [*****]                $[***]/liter

Beginning [**************], each of the foregoing prices may be changed by
multiplying such price by a fraction, the numerator of which shall be the
Index as of the most recent date available prior to the date of such change
and the denominator of which shall be the Index as of the most recent date
available prior to the date of the Agreement to which this Schedule B is
attached. As used herein, "Index" shall mean the Producer Price Index
published by the U.S. Department of Labor.

ROYALTY RATES:

For Product Purchased for Use in Human In Vitro Diagnostics or Pharmaceutical
Therapeutics Research and Development

<TABLE>
<S>                                               <C>
The first $[*********] of Net Sales:              the greater of: [***]% of Net Sales of
                                                  Tests, or $[****] per Test.
Thereafter:                                       the greater of [***]% of Net Sales of
                                                  Tests, or $[****] per Test.
</TABLE>

For Product Purchased for Pharmaceutical Screening

                          [***]% of Net Sales of Tests



<PAGE>   3

THIS EXHIBIT HAS BEEN REDACTED AND IS THE SUBJECT OF A CONFIDENTIAL TREATMENT
REQUEST. REDACTED MATERIAL IS MARKED WITH "*" AND BRACKETS AND HAS BEEN FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION.

IN WITNESS WHEREOF, the undersigned have executed and delivered this Amendment
by their duly authorized representatives as of 5/14/1998.


<TABLE>
<CAPTION>
TROPIX, INC.                            DIGENE CORPORATION
<S>                                     <C>
By: /s/Michael A. Nemzek                By: /s/Donna Marie Seyfried
   -----------------------------           ----------------------------------------

Name: Michael A. Nemzek                 Name: Donna Marie Seyfried
      --------------------------             --------------------------------------

Title: Vice President, Marketing        Title: Vice President, Business Development
      --------------------------              -------------------------------------

Date: May 14,1998                       Date: May 14, 1998
      --------------------------             --------------------------------------
</TABLE>

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