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                                                                  EXHIBIT 10.21B

                    [Abbott Diagnostics Division Letterhead]



December 22, 1997

Mark Samuels
President & CEO
SpectRx, Inc.


Dear Mark:

This letter serves three purposes with respect to the ongoing Abbott/SpectRx
research program for the extraction of interstitial fluid for glucose
monitoring: summarizes our mutual understanding of the current state of the
feasibility phase, outlines the key next steps the parties will take and serves
to amend certain terms (the "Amendment") of our Research & Development and
License agreement dated October 10, 1996 (the "Agreement").

1.      The state of the feasibility phase at this time is best summarized as
        follows:

        A.     SpectRx has made progress in the [ * ] extraction of
               [ * ] of interstitial fluid, [ * ] in the
               Agreement.

        B.     The Research Program has made progress in the extraction of
               interstitial fluid by using [ * ] (the "Current Technology").

        C.     The Current Technology [ * ] at this time and [ * ] to 
               understand the best [ * ] of technology options and whether
               this [ * ] provides acceptable performance, both from a
               technical and user acceptability perspective, while meeting 
               [ * ] for a possible commercial product (the "Optimization").

        D.     SpectRx has [ * ] in an effort to [ * ] as described in the 
               Agreement and will [ * ].

2.      The parties acknowledge the Research Program is not complete and to best
        deal with the current situation, they will proceed in the following
        manner and accordingly, the Agreement shall be amended as follows:

        A.     As part of the Optimization, user studies will be performed to 1)
               get additional interstitial fluid extraction performance data on
               the various technology permutations of the Current Technology,
               and 2) understand the user acceptance of the Current Technology
               alternatives from a [ * ] over current finger stick technology. 
               These studies, as well as additional steps necessary to complete
               the Optimization including research, drafting protocols, 
               internal studies, additional clinicals, focus panels and 
               resource assignments [ * ]

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[ * ] Certain information on this page has been omitted and filed separately
with the Commission. Confidential treatment has been requested with respect to
the omitted portions. 


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               [ * ]  will be decided on and executed under the direction of 
               Abbott with input from SpectRx. In addition, ongoing [ * ] will
               be completed by [ * ] [ * ] and sections of the [ * ]
               considered appropriate by Abbott will be shared with SpectRx.
               [ * ] 

        B.     [ * ] of the Optimization, [ * ] the parties may decide to 
               re-direct the Research Program, as defined in the Agreement, 
               based on the results of the Optimization. If a re-direction is 
               mutually agreed to, the parties will negotiate in good faith to
               restructure the Research Program Plan, milestones and other 
               terms of the Agreement which must be re-structured as 
               necessitated by the revised Research Program Plan, and all 
               terms necessary for any new business relationship desired by 
               the parties. Notwithstanding the foregoing, in no event will 
               the total funds for the research milestones, the total project 
               milestones or royalties be increased and there will be no 
               payment due from Abbott for the restructuring itself.

3.      In recognition of the progress SpectRx has made in the [ * ]
        extraction of [ * ] of interstitial fluid, the parties agree:

        A.     Abbott will make a one time $500,000 progress payment to SpectRx
               within fifteen (15) days of execution of this Amendment. This
               payment is not a Research Program milestone payment under the
               Agreement and this payment is not to be construed as evidencing
               achievement of any milestone under the Agreement. 
               
        B.     [ * ].

        C.     The provisions of this Amendment are hereby made a part of the
               Agreement; any conflict between the provisions of this Amendment
               and the Agreement shall be resolved in favor of the provisions of
               this Amendment. All capitalized terms used in this Amendment and
               not defined herein shall have the same meanings as given to them
               in the Agreement. Except as specifically set forth in this
               Amendment, nothing herein shall be construed as a modification or
               waiver by Abbott or SpectRx of any rights or remedies that Abbott
               or SpectRx may have under the Agreement, including without
               limitation, any rights to terminate the Agreement.

Please countersign and return one copy of this letter as confirmation of
SpectRx's agreement to all statements and amended terms herein.

Sincerely,                                         Agreed and accepted,


    /s/ JIM KOZIARZ                                     /s/ MARK A. SAMUELS
- -------------------------------                    -----------------------------
Jim Koziarz                                        Mark Samuels
Corporate Vice President                           President & CEO
Diagnostics Products R&D                           SpectRx, Inc.
Abbott Laboratories

       12/23/97                                            12/25/97
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Date                                               Date

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[ * ] Certain information of this page has been omitted and filed separately
with the Commission. Confidential treatment has been requested with respect to
the omitted portions.