Exhibit 10.20

                              [Covance Letterhead]

10 November 1998


Mr. Christopher Kuebler
[address]        

        Re:    Amendment No. 1 to Employment Agreement between
               Christopher Kuebler and Covance Inc.

Dear Chris:

Please refer to that certain Employment Agreement between you and Covance Inc.
dated as of November 1, 1996 (the "Employment Agreement"). This Letter Agreement
will constitute Amendment No. 1 to the Employment Agreement and will amend the
Employment Agreement as follows:

1.      Section IX(d) shall be amended to read in its entirety as follows:

        "(d) Change-of-Control: In the event of an Event of Termination (as
        defined below), Executive will be entitled to receive all of the
        "Severance Benefits" described in paragraph (c) above, and, in addition:

               (i) All stock options (including the Stock Options), restricted
        stock (including the Restricted Stock), deferred compensation and
        similar benefits which have not become vested on the date of an Event of
        Termination shall become vested upon such Event.

               (ii) The Executive shall be entitled to receive any payments
        calculated pursuant to Section XVIII hereof.

               (iii) In the event you are involved in any dispute about your
        rights or obligations under this Agreement arising on or after a
        Change-of-Control, the Company shall pay all legal costs and fees
        incurred by you in connection with such dispute promptly upon receipt of
        any invoice relating thereto.

               (iv) The benefits set forth in Sections VIII(c) and VIII(d)
        hereof and medical, dental, disability and life insurance will be
        continued, to the extent they are not otherwise prohibited under the
        respective plans, until you find other employment but not longer than
        three years from the date of the Event of Termination.

        For the purposes of this Agreement, an Event of Termination is defined
        to be a termination of Executive's employment by the Company (for
        reasons other than Cause)

<PAGE>

        or a Constructive Termination (as defined below) of Executive's
        employment, in each case within 24 months following a
        Change-of-Control (as defined below), or Executive's voluntary
        termination of his employment for any reason or no reason during the
        one-month period commencing twelve months following a
        Change-of-Control and ending thirteen months after such
        Change-of-Control (a "Voluntary Termination"); provided, however, a
        Voluntary Termination shall not be an Event of Termination if it
        arises from a Change-of-Control pursuant to subsection (iv) under the
        definition of Change-of-Control unless the tender offer or exchange
        offer is a tender or exchange offer for securities representing 20% or
        more of the combined voting power of Covance's then outstanding
        securities.

        For purposes of this Agreement, a Change-Of-Control is defined to occur
        when:

               (i) any person (including as such term is used in Section 13(d)
        and 14(d)(2) of the Securities Exchange Act of 1934) becomes the
        beneficial owner, directly or indirectly, of Company securities
        representing 20% or more of the combined voting power of the Company's
        then outstanding securities; or

               (ii) as a result of a proxy contest or contests or other forms of
        contested shareholder votes (in each case either individually or in the
        aggregate), a majority of the individuals elected to serve on the
        Company's Board of Directors are different than the individuals who
        served on the Company's Board of Directors at any time within the two
        years prior to such proxy contest or contests or other forms of
        contested shareholder votes; or

               (iii) the Company's shareholders approve a merger or
        consolidation (where in each case the Company is not the survivor
        thereof), or a sale or disposition of all or substantially all of the
        Company's assets or a plan of partial or complete liquidation; or

               (iv) an offeror (other than the Company) purchases shares of the
        Company's common stock pursuant to a tender or exchange offer for such
        shares.

        For purposes of this Agreement, a Constructive Termination is defined to
        be:

               (i) a material breach by the Company of this Agreement,
        including, without limitation, a reduction in your then current salary
        or the percentage of base salary eligible for incentive compensation;

               (ii) a diminution of Executive's responsibilities, status, title
        or duties under this Agreement;

               (iii) a relocation of Executive's work place which increases the
        distance between Executive's principal residence and Executive's work
        place by more than 25 miles;

               (iv) a failure by the Company to provide Executive with benefits
        which are as favorable to Executive in all material respects as those
        provided immediately prior to the Change-of-Control; or

                                                                               2
<PAGE>

               (v) the failure of any acquirer or successor in interest to the
        business of the Company to agree in writing to be bound by the terms of
        this Agreement within four months of any Change-of-Control."

2. Section IX(f) shall be amended to read in its entirety as follows:

        If there has been an Event of Termination or if there has been no
        Change-of-Control but Executive has been terminated without Cause, the
        Company shall provide for Executive, at the Company's cost, executive
        outplacement support for one year following such termination.

3. Section XVII(i) shall be amended to read in its entirety as follows:

        If there has been an Event of Termination or if there has been no
        Change-of-Control but Executive has been terminated without Cause, the
        obligation of the Company to make to Executive any or all of the
        payments specified under this Agreement (including, without limitation,
        the payments specified in Section IX) shall be subject to Executive's
        execution and delivery to the Company of a release in form and substance
        reasonably satisfactory to the Company of all claims, demands, suits, or
        actions, whether in law or at equity, Executive has or may have relating
        to or giving rise from such Event of Termination or non-Cause
        termination.

If the foregoing meets with your understanding of our Agreement, please so
indicate by signing this Agreement below.

Very truly yours,

COVANCE INC.



By:                                 
   ------------------------
   Jeffrey S. Hurwitz
   Corporate Senior Vice President

ACCEPTED AND AGREED:

By:
   ------------------------
   Christopher A. Kuebler