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                                                                    EXHIBIT 10.1


                             AMENDMENT NUMBER ONE TO
                              AGREEMENT AND RELEASE

     THIS AMENDMENT NUMBER ONE TO AGREEMENT AND RELEASE (the "Amendment") by and
between Security Dynamics Technologies, Inc. (the "Company") and D. James Bidzos
(the "Consultant") is made this 20th day of July, 1999 (the "Effective Date").

                                    RECITALS

     WHEREAS, the Company and the Consultant are parties to that certain
Agreement and Release dated as of February 18, 1999 (the "Agreement"); and

     WHEREAS, the parties desire to amend the Agreement to extend the exercise
date of certain of the Consultant's stock options.

     NOW THEREFORE, in reliance on the foregoing Recitals and in consideration
of the mutual consideration recited therein and contained herein, the parties
agree as follows:

1.   OPTION EXERCISE DATE. Section 2(c) of the Agreement is deleted, and
     replaced with the following new Section 2(c):

     (c)  EXERCISE OF OPTIONS. The Consultant must exercise: (i) fifty percent
          (50%) of his stock options described in Section 2(a) of the Agreement
          on the last to occur of: (A) September 30, 1999, or (B) the effective
          date of the Financial Standards Accounting Board's ("FASB's")
          interpretation of AICPA Accounting Principles Board Opinion (APB) 25;
          PROVIDED HOWEVER that in the event that the FASB does not issue its
          interpretation of APB 25 on or before December 31, 1999, then the
          Consultant must exercise those options on December 31, 1999; and (iii)
          the remaining fifty percent (50%) of his unexercised stock options on
          or before December 31, 1999.

2.   COUNTERPARTS. This Amendment may be executed in counterparts, each of which
     shall be an original and together which shall constitute one and the same
     instrument.

3.   EFFECT OF AMENDMENT. This Amendment amends the Agreement as of the
     Effective Date, and except as amended hereby, the Agreement shall continue
     in full force and effect.

IN WITNESS WHEREOF, the parties have executed this Amendment as of the date
first above written.

SECURITY DYNAMICS TECHNOLOGIES, INC.

By: /s/ Charles R. Stuckey, Jr.               /s/ D. James Bidzos
    ---------------------------------         ---------------------------------
    Name:   Charles R. Stuckey, Jr.           D. James Bidzos
    Title:  Chairman of the Board and
            Chief Executive Officer