Document:

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                                                                  EXHIBIT 10.1.6

                               TENTH AMENDMENT TO
                        AGREEMENT OF LIMITED PARTNERSHIP
                      OF EOP OPERATING LIMITED PARTNERSHIP

     THIS TENTH AMENDMENT TO AGREEMENT OF LIMITED PARTNERSHIP OF EOP OPERATING
LIMITED PARTNERSHIP (this "AMENDMENT"), dated October 1, 1999, is entered into
by EQUITY OFFICE PROPERTIES TRUST, a Maryland real estate investment trust, as
managing general partner (the "GENERAL PARTNER") of EOP Operating Limited
Partnership, a Delaware limited partnership (the "PARTNERSHIP"), for itself and
on behalf of itself and the Limited Partners of the Partnership, and that or
those persons whose signatures are set forth on the signature pages attached
hereto.

     WHEREAS, on the date hereof, PALO ALTO SQUARE LIMITED PARTNERSHIP, an
Illinois limited partnership ("CONTRIBUTOR"), is receiving 1,012,623 Class B
Units of limited partnership interest in the Partnership ("OP UNITS") in
exchange for certain real property interests, pursuant to a closing under, and
as more particularly described in, that certain Contribution Agreement dated
September 28, 1999, by and between the General Partner (or its permitted
assignees) and Contributor (the "CONTRIBUTION AGREEMENT").  The Contributor is
the owner of certain property (the "PROPERTY") described in the Contribution
Agreement.

     WHEREAS, Contributor has assigned the OP Units to its constituent partners
as set forth on Schedule 1 who desire to become Limited Partners of the
Partnership (the "EQUITY HOLDERS").

     WHEREAS, pursuant to the authority granted to the General Partner under
the Agreement of Limited Partnership of the Partnership dated as of July 3,
1997, as amended thereafter (as so amended, the "PARTNERSHIP AGREEMENT"), the
General Partner desires to amend the Partnership Agreement to reflect (i) the
admission of the Contributor as an Additional Limited Partner, (ii) the
admission of the Equity Holders as Substituted Limited Partners and the holders
of the OP Units and (iii) certain other matters described herein.

     WHEREAS, the Contributor and the Equity Holders desire to become a party
to the Partnership Agreement and to be bound by all of the terms, conditions
and other provisions of this Amendment and the Partnership Agreement.

     NOW, THEREFORE, in consideration of the premises and for other good and
valuable consideration, the receipt and sufficiency of which hereby are
acknowledged, the General Partner hereby amends the Partnership Agreement as
follows:

     1.    ADMISSION TO PARTNERSHIP.  The Contributor is hereby admitted to the
Partnership as an Additional Limited Partner in accordance with Section 12.2 of
the Partnership Agreement and hereby agrees to become a party to the
Partnership Agreement and to be bound by all of the terms, conditions and other
provisions of the Partnership Agreement, including, but not limited to, the
power of attorney set forth in Section 15.11 of the Partnership Agreement. The
General Partner hereby consents to the transfer of the OP Units from the

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Contributor to the Equity Holders as reflected on Schedule 1 attached hereto.
Accordingly, the Equity Holders are hereby admitted to the Partnership as
Substituted Limited Partners in accordance with Section 11.4 of this
Partnership Agreement and hereby agree to become parties to the Partnership
Agreement and to be bound by all of the terms, conditions and other provisions
of the Partnership Agreement (as amended by this Amendment), including, but not
limited to, the power of attorney set forth in Section 15.11 of the Partnership
Agreement, and the Contributor is no longer a Limited Partner of the
Partnership.

     2.    RESTATEMENT OF EXHIBIT A.  Exhibit A to the Partnership Agreement
hereby is amended by replacing such Exhibit A with Exhibit "A" attached to this
Amendment.

     3.    RIGHT TO ASSIGN.  Notwithstanding any other provision of this
Amendment or of the Partnership Agreement, the Equity Holders shall have the
right to assign all or any portion of their OP Units, together with any and all
other rights of the Equity Holders pursuant to this Amendment or the
Partnership Agreement, to one or more of the constituent partners or
shareholders, members, partners or beneficiaries of constituent partners of the
Equity Holders on the date hereof, whether direct or indirect, without the need
for the consent of the Managing General Partner or any other General Partner or
Limited Partner and without being subject to the right of first refusal set
forth in Section 11.3.A(a) of the Partnership Agreement, but in each case
subject to the restrictions and conditions set forth in Sections 11.3.C,
11.3.D, 11.3.E, 11.6.E and 11.6.F of the Partnership Agreement.  Upon the
delivery of written notice of such an assignment to the Managing General
Partner, each assignee of OP Units pursuant to the immediately preceding
sentence shall be admitted to the Partnership as a Substituted Limited Partner
owning the OP Units so assigned and having all of the rights of a Limited
Partner under the Partnership Agreement and this Amendment, subject only to
such assignee executing and delivering, to the Partnership an acceptance of all
of the terms and conditions of the Partnership Agreement and such other
documents or instruments as the Managing General Partner may reasonably require
to effect such admission, in accordance with Section 11.4.B of the Partnership
Acreement.  Each permitted assignee of any of the OP Units, issued to the
Contributors pursuant to the Contribution Agreement and subsequently
transferred to the Equity Holders, that is admitted as a Substituted Limited
Partner in accordance with this Section 3 or Article XI of the Partnership
Agreement, for so long as such Person owns any such OP Units, is referred to in
this Amendment as an "Indirect Equity Holder."  Upon satisfaction of the
condition described in the second sentence of this Section 3, the Managing
General Partner shall amend Exhibit A to the Partnership Agreement in the manner
described in Section 11.4.C of the Partnership Agreement.  For purposes of
Section 8.6 of the Partnership Agreement, each Equity Holder and Indirect Equity
Holder shall be entitled to exercise its right to require the Partnership to
redeem all or any portion of the OP Units assigned to it by an Equity Holder at
any time on or after the date hereof.

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     4.    ADJUSTMENTS TO CARRYING VALUES.

           (a)   The Carrying Values of the Assets of the Partnership shall be
adjusted in accordance with the procedures described in Section 1.D of Exhibit
B to the Partnership Agreement:  provided, however, that in order to minimize
the administrative burden associated with the adjustments required by this
Section 4(a) in connection with the distribution of the Cash Amount to an Equiy
Holder, or an Indirect Equity Holder, the Partnership shall make the
adjustments to the Carrying Values of the Partnership's Assets (and the
resulting adjustments to the Capital Accounts of the Partners) only upon the
happening of the most material event during the calendar year that is described
in Section 1.D(2) of Exhibit B to the Partnership Agreement (the "ANNUAL
ADJUSTMENT"); and provided further, that upon the distribution of the Cash
Amount to an Equity Holder or an Indirect Equity Holder or, at the option of
the General Partner, upon the occurrence of any other event described in
Section 1.D(2) of Exhibit B to the Partnership Agreement, that occurs during
any year other than as of the date of the Annual Adjustment, the Partnership
shall, at the time of such distribution, make adjustments to the Carrying
Values of the Partnership's Assets in accordance with the procedures described
in Section 1.D of Exhibit B to the Partnership Agreement for purposes of
adjusting the Capital Account of an Equity Holder, or such Indirect Equity
Holder who has exercised his Redemption Right or such other affected Partner,
but no such adjustments shall be necessary at such time with respect to the
Capital Account balances of Partners who remain Partners through the date of
the Annual Adjustment or are otherwise not directly affected by any such other
event.

           (b)   Any determination of the fair market value of Partnership
assets pursuant to Section 1.D of Exhibit B to the Partnership Agreement (for
purposes of calculating Unrealized Gain or Unrealized Loss) with respect to
adjusting the Carrying Values of Partnership assets in connection with the
exercise of Redemption Rights by an Equity Holder or any Indirect Equity Holder
shall be made by assuming that the aggregate fair market value of all
Partnership assets is equal to the aggregate Cash Amount that would be
distributed by the Partnership if all Partnership Units held by all Partners
(including the General Partners) were redeemed in exchange for the Cash Amount
with respect to each such Partnership Unit at such time, provided, however,
such valuation methodology shall not be utilized for purposes of determining the
fair market  value of the Partnership's assets with respect to any such exercise
of redemption Rights in contemplation of an assignment by or reorganization of
the Partnership for the benefit of creditors and any liquidation of the
Partnership related thereto or following the filing by (or in contemplation of a
filing) by the Partnership of a case under Title 11 of the U.S. Code.

     5.    ALLOCATIONS.  Nothwithstanding the provisions of Section 2.C of
Exhibit C to the Partnership Agreement, for purposes of allocating items of
income, gain, loss and deduction with respect to the Property in the manner
required by Section 704(c) of the Code, the Partnership shall employ, and shall
cause any entity controlled by the Partnership which holds title to any of the
Property to employ, the "traditional method" as set forth in Regulation Section
1.704-3(b).

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<PAGE>   4

     6.    OBLIGATION TO RESTORE DEFICIT CAPITAL ACCOUNTS.

           (a)   For purposes of this Section 6, the following terms shall have
the meanings set forth below:

                 (i)      "Aggregate Protected Amount" means the aggregate
                 balances of the Protected Amounts, if any, of all Protected
                 Partners, as determined on the date in question.

                 (ii)     "Protected Amount" means, with respect to any
                 Protected Partner, an amount equal to (i) the taxable gain, if
                 any, that would be realized by such Protected Partner if such
                 Partner were to dispose of its Partnership Interests for no
                 consideration other than the release or deemed release of
                 liabilities of the Partnership assumed by or otherwise
                 allocable to such Partner under Code Section 752, as such
                 hypothetical gain is determined from time to time, less (ii)
                 such Partner's share of "qualified nonrecourse financing" as
                 defined in Code Section 465(b)(6) and the Treasury Regulations
                 thereunder, as such share is determined from time to time in
                 accordance with Treasury Regulations Section 1.752-3(a).  The
                 Protected Amount allocable to any Protected Partner may be
                 modified from time to time by an amendment to the Agreement or
                 by execution of a written instrument by and between such
                 Protected Partner and the General Partner, acting on behalf of
                 the Partnership and without the prior written consent of any
                 other Partner (whether or not Protected Partners).

                 (iii)    "Protected Partner(s)" means that or those Limited
                 Partner(s) designated as Protected Partner(s) on Exhibit A
                 attached hereto and made a part hereof, as such designation
                 may be modified from time to time by the General Partner,
                 whether by express amendment to this Agreement or by execution
                 of a written instrument by and between any Protected
                 Partner(s) and the General Partner, acting on behalf of the
                 Partnership and without the prior consent of other Limited
                 Partners (whether or not Protected Partners). For purposes
                 hereof, any successor, assignee, or transferee of Partnership
                 Interests from a Protected Partner, which successor, assignee
                 or transferee determines its basis in such Units by reference
                 to the basis of the predecessor, assignor or transferor
                 Protected Partner, shall be considered a Protected Partner for
                 purposes hereof.

                 (iv)     "Recourse Liabilities" means the amount of
                 liabilities owed by the Partnership (other than Nonrecourse
                 Liabilities and liabilities to which Partner Nonrecourse
                 Deductions are attributable in accordance with Treasury
                 Regulations Section 1.704-2(i)).

           (b)   Notwithstanding any provision of the Partnership Agreement to
the contrary (including, without limitation, the second sentence of Section
13.3 of the Partnership

                                       4
<PAGE>   5
Agreement), upon liquidation of the Partnership or upon the liquidation of the
Partnership Interest of a Protected Partner, such Protected Partner whose
interest is being liquidated shall contribute to the Partnership in accordance
with Treasury Regulation Section 1.704-1(b)(2)(ii)(b)(2), the deficit balance,
if any, in its Capital Account, calculated after the allocation for such year of
all items of Net Income, Net Losses, Gross Income and Unrealized Gain or
Unrealized Loss allocated in accordance with Section 6.1 of the Partnership
Agreement, Section l.D of Exhibit B to the Partnership Agreement, and modified
by Section 7 of this Amendment; provided, however, that in no event shall such
contribution obligation for any Protected Partner exceed such Protected
Partner's Protected Amount.  The obligation created pursuant to this Section
6(b) shall be for the benefit of the Partnership, its general partners (the
"GENERAL PARTNERS"), the creditors of the Partnership or any other person to
whom any debts, liabilities or obligations are owed by (or who otherwise has any
claim against) the Partnership or the General Partners in their capacities as
general partners of the Partnership and shall be enforceable by such parties.
Each Protected Partner unconditionally and irrevocably waives any subrogation,
reimbursement or similar rights to which it might otherwise be entitled as the
result of its performance with respect to the obligation created pursuant to
this Section 6(b), whether such rights arise with respect to the Partnership,
another Partner of the Partnership or a third party; provided, however, that the
General Partners shall in all events be entitled to enforce the contribution
obligation of a Protected Partner undertaken pursuant to Section 6(b).
Notwithstanding anything herein to the contrary, the obligation of a Protected
Partner to contribute amounts pursuant to this paragraph shall continue in full
force and effect in accordance with the terms hereof until one year following
the transfer or other disposition by such Protected Partner of its Partnership
Interests unless the Partnership and such Protected Partner agree otherwise.
Unless expressly agreed by the parties to the contrary in this or an other
agreement, any Partner of the Partnership who is not a Protected Partner shall
have no obligation to contribute amounts to the Partnership.

     7.    ALLOCATIONS OF NET INCOME AND NET LOSSES.

           For purposes of maintaining Capital Accounts and in determining the
rights of the Partners among themselves, Partnership income, gain, loss and
deductions (computed in accordance with Exhibit B of the Partnership Agreement)
shall be allocated among the Partners in each taxable year (or portion thereof)
as provided in Section 6.1 of the Partnership Agreement, subject to the
following adjustments:

           (a)   Net Income.  After giving effect to the special allocations
set forth in Section 1 of Exhibit C of the Partnership Agreement, Net Income
otherwise allocable to the General Partners pursuant to the provisions of
Section 6.1.A(i) of the Partnership Agreement shall be allocated:

                 (i)      first, to the General Partners to the extent that
                 cumulative Net Losses previously allocated the General
                 Partners pursuant to Section 7(b)(iii) of this Amendment
                 exceed cumulative Net Income previously allocated to the
                 General Partners pursuant to this clause (i);

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<PAGE>   6

                 (ii)     next, to each Protected Partner until the cumulative
                 Net Income allocated such Protected Partner under this clause
                 (ii) equals the cumulative Net Losses allocated such Protected
                 Partner under Section 7(b)(ii) of this Amendment (and, within
                 the class of Protected Partners, pro rata in proportion to
                 their respective percentages of the cumulative Net Losses
                 allocated all Protected Partners pursuant to Section 7(b)(ii)
                 hereof); and

                 (iii)    thereafter, to the General Partners until the
                 cumulative Net Income allocated under this clause (iii) equals
                 the cumulative Net Losses allocated the General Partners under
                 Section 7(b)(i) of this Amendment.

         (b)     Net Losses.  After giving effect to the special allocations
set forth in Section 1 of Exhibit C of the Partnership Agreement, Net Losses
otherwise allocable to the General Partners pursuant to the last sentence of
Sections 6.1.B of the Partnership Agreement shall be allocated:

                 (i)      first, to the General Partners in an amount equal to
                 the excess of (a) the amount of Recourse Liabilities over (b)
                 the Aggregate Protected Amount;

                 (ii)     next, to and among the Protected Partners, in
                 proportion to their respective Protected Amounts, until such
                 time as the Protected Partners have been allocated cumulative
                 Net Losses pursuant to this clause (ii) equal to the Aggregate
                 Protected Amount; and

                 (iii)    thereafter, to the General Partners.

         8.      AMENDMENTS.  Notwithstanding any provision in the Partnership
Agreement to the contrary, the provisions of this Amendment may be waived or
amended or otherwise modified with the prior written consent of holders (being
either the Equity Holders and/or the Indirect Equity Holders) of more than
seventy-five percent (75%) of the OP Units at the time outstanding and without
the consent of any other Limited Partner.

         9.      TIME IS OF THE ESSENCE.  Time is of the essence of each and
                 every provision of this Amendment.

         10.     DEFINED TERMS.  All capitalized terms used in this Amendment
and not otherwise defined shall have the meanings assigned to them in the
Partnership Agreement.  Except as modified herein, all terms and conditions of
the Partnership Agreement shall remain in full force and effect, which terms
and conditions the General Partner hereby ratifies and affirms.

                                       6
<PAGE>   7

         IN WITNESS WHEREOF, the undersigned has executed this Amendment as of
the date first set forth above.

                                      EQUITY OFFICE PROPERTIES TRUST, a Maryland
                                      real estate investment trust, a General
                                      Partner of EOP Operating Limited
                                      Partnership and on behalf of existing
                                      Limited Partners

                                      By:        /s/ STANLEY M. STEVENS
                                                 -------------------------------
                                      Name:      Stanley M. Stevens
                                      Title:     Executive Vice President

                              CONTRIBUTOR:

                              PALO ALTO SQUARE LIMITED PARTNERSHIP, an Illinois
                              limited partnership, by its general partner:

                                       Samuel Zell, Robert Lurie General
                                       Partners, an Illinois partnership, by one
                                       of its general partners:

                                               Zell General Partners, Inc., an
                                               Illinois corporation

                                               By:    /s/ DONALD J. LIEBENTRITT
                                                      --------------------------
                                               Name:  Donald J. Liebentritt
                                               Title: Executive Vice President

                                       7
<PAGE>   8

SIGNATURES CONTINUED

                                                   EQUITY HOLDERS:

                                                   /s/ ADRIENNE NASSAU
                                                   -----------------------------
                                                   Adrienne Nassau
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE: /s/ AN
                                                                 ---------------

                                                   ANDA Partnership

                                                   By:   /s/ MARK SLEVAK
                                                         -----------------------
                                                         Its: Trustee
                                                             -------------------
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE: /s/ MS
                                                                 ---------------

                                       8

<PAGE>   9

SIGNATURES CONTINUED

                                                   /s/ EDWARD H. SCHWARTZ
                                                   -----------------------------
                                                   Edward H. Schwartz
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE:
                                                                 ---------------

                                                   Estate of Sylvia K. Grossman

                                                   By:   /s/
                                                         -----------------------
                                                         Its:
                                                              ------------------
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE:
                                                                 ---------------

                                       9
<PAGE>   10

SIGNATURES CONTINUED

                                      Henry A. Proesel Descendants Trusts
                                      Partnership

                                      By:   /s/ MARGARET E. MCNAMARA, Co-Trustee
                                            ------------------------------------
                                            Its:  Partner
                                                  ------------------------------
                                      TO BECOME A PROTECTED PARTNER
                                      INITIAL HERE: /s/ MEM
                                                    ----------------------------

                                      Hugh M. Hefner 1991 Trust

                                      By:      /s/ HUGH HEFNER
                                               ---------------------------------
                                               Its:
                                                    ----------------------------
                                      TO BECOME A PROTECTED PARTNER
                                      INITIAL HERE:
                                                     ---------------------------

                                      Isabelle Kole Stein Trust

                                      By:      /s/ ISABELLE KOLE STEIN
                                               ---------------------------------
                                               Its:
                                                    ----------------------------
                                      TO BECOME A PROTECTED PARTNER
                                      INITIAL HERE: /s/ IKS
                                                    ----------------------------

                                      10
<PAGE>   11

SIGNATURES CONTINUED

                                      James V. Proesel Marital Exempt Trust

                                      By:   /s/ MARGARET E. MCNAMARA, Co-Trustee
                                            ------------------------------------
                                            Its:  Co-Trustee
                                                --------------------------------
                                      TO BECOME A PROTECTED PARTNER
                                      INITIAL HERE:  /s/ MEM
                                                     ---------------------------

                                       /s/ JUDITH MOLINSKY
                                      ------------------------------------------
                                      Judith Molinsky
                                      TO BECOME A PROTECTED PARTNER
                                      INITIAL HERE:  /s/ JM
                                                     ---------------------------

                                       11
<PAGE>   12

SIGNATURES CONTINUED

                                                   LFT Partnership

                                                   By:  /s/ ANN LURIE
                                                        ------------------------
                                                        Its:  Trustee
                                                              ------------------
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE: /s/ AL
                                                                 ---------------

                                                   /s/ MORTON H. FRIEDMAN
                                                   -----------------------------
                                                   Morton H. Friedman
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE:  /s/ MHF
                                                                  --------------

                                                   /s/ MORTON S. GROSSMAN
                                                   -----------------------------
                                                   Morton S. Grossman
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE:  /s/ MSG
                                                                  --------------

                                       12
<PAGE>   13

SIGNATURES CONTINUED

                                                   Robert H. & Ann Lurie Trust

                                                   By: /s/ MARK SLEZAK
                                                       -------------------------
                                                       Its: Trustee
                                                            --------------------
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE: /s/ MS
                                                                 ---------------

                                                   Robert Scot Bldg Venture

                                                   By: /s/ ROBERT HARRIS
                                                       -------------------------
                                                       Its: TTEE
                                                            --------------------
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE: /s/ RH
                                                                 ---------------

                                                  RSB Properties Trust

                                                   By: /s/
                                                       -------------------------
                                                       Its: Trustee
                                                            --------------------
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE: /s/
                                                                 ---------------

                                                  Seiler Trust UTA dated 3/15/95

                                                   By: /s/ DONALD H. SEILER
                                                       -------------------------
                                                       Its: Trustee
                                                            --------------------
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE: /s/ DS
                                                                 ---------------

                                       13
<PAGE>   14

SIGNATURES CONTINUED

                                    Seiler Family Revocable Trust dated 9/18/90

                                    By:  /s/ STUART W. SEILER
                                         ---------------------------------------
                                         Its: Trustee
                                         ---------------------------------------
                                    TO BECOME A PROTECTED PARTNER
                                    INITIAL HERE: /s/ SWS
                                                  ------------------------------

                                    Helen S. Fredkin Revocable Trust

                                    By:  /s/ HELEN S. FREDKIN REV. TRUST
                                         ---------------------------------------
                                         Its:  Trustee
                                              ----------------------------------
                                    TO BECOME A PROTECTED PARTNER
                                    INITIAL HERE: /s/ HSF
                                                  ------------------------------

                                       14
<PAGE>   15

SIGNATURES CONTINUED

                                                   Sgm & Pin Trust

                                                By:   /s/ STUART MOLDAW
                                                      --------------------------
                                                      Its:
                                                           ---------------------
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE:  /s/ SM
                                                                  --------------

                                                   Steven Kole Trust

                                                By:   /s/ STEVEN M. KOLE
                                                      --------------------------
                                                      Its: Trustee
                                                           ---------------------
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE:  /s/ SMK
                                                                  --------------

                                                By:   /s/ SUSAN HANDELMAN MILLER
                                                      --------------------------
                                                      Susan Handelman Miller
                                                   TO BECOME A PROTECTED PARTNER
                                                   INITIAL HERE: /s/ SHM
                                                                 ---------------

                                                By:   /s/ VICTOR A. LOWNES
                                                      --------------------------
                                                      Victor A. Lownes, III
                                                   TO BECOME A PROTECTED PARTNER
                                                    INITIAL HERE: /s/ VAL
                                                                  --------------

                                       15
<PAGE>   16

SIGNATURES CONTINUED

                                     Samstock/Alpha, L.L.C.

                                     By: /s/ DONALD J. LIEBENTRITT
                                         ---------------------------------------
                                     Its: Vice President
                                         ---------------------------------------
                                     TO BECOME A PROTECTED PARTNER
                                     INITIAL HERE:   /s/ DJL
                                                    ----------------------------

                                     Samstock/SZRT, L.L.C.

                                     By:   /s/ DONALD J. LIEBENTRITT
                                           -------------------------------------
                                           Its:  Vice President
                                                --------------------------------
                                     TO BECOME A PROTECTED PARTNER
                                     INITIAL HERE:   /s/ DJL
                                                    ----------------------------

                                     Samstock/ZFT, L.L.C.

                                     By: /s/ DONALD J. LIEBENTRITT
                                         ---------------------------------------
                                     Its: Vice President
                                         ---------------------------------------
                                     TO BECOME A PROTECTED PARTNER
                                     INITIAL HERE:   /s/ DJL
                                                    ----------------------------

                                     Samstock/ZGPI, L.L.C.

                                     By: /s/ DONALD J. LIEBENTRITT
                                         ---------------------------------------
                                     Its: Vice President
                                         ---------------------------------------
                                     TO BECOME A PROTECTED PARTNER
                                     INITIAL HERE:   /s/ DJL
                                                    ----------------------------

                                       16<PAGE>   1
                                                                    Exhibit 10.1

                              EMPLOYMENT AGREEMENT

       THIS EMPLOYMENT AGREEMENT (this "Agreement"), dated as of March 14, 2000,
by and between Alteon Inc., a Delaware corporation (the "Company"), and Robert
deGroof, Ph.D. (the "Employee").

       WHEREAS, the Company wishes to employ the Employee as Senior Vice
President for Scientific Affairs; and

       WHEREAS, the Employee wishes to enter into the employ of the Company as
its Senior Vice President for Scientific Affairs;

       NOW, THEREFORE, in consideration of the premises and the mutual covenants
contained herein, the parties hereby agree as follows;

       1.     Term of Employment. Subject to the terms and conditions hereof,
              the Company will employ the Employee, and the Employee will serve
              the Company, as Senior Vice President for Scientific Affairs for a
              period beginning on the date hereof and terminating three (3)
              years from such date, subject to extension by mutual agreement of
              the Company and the Employee (such term, as it may be extended, is
              hereinafter referred to as the "Term of Employment").

       2.     Duties. During the Term of Employment, the Employee will serve as
              Senior Vice President for Scientific Affairs, subject to the terms
              of this Agreement and the direction and control of the Board of
              Directors and/or the Chief Executive Officer of the Company. The
              Employee will, during the Term of Employment, serve the Company
              faithfully, diligently and competently and to the best of his
              ability, and will, consistent with the dignity of Senior Vice
              President for Scientific Affairs of the Company, hold, in addition
              to the offices of Senior Vice President of Scientific Affairs of
              the Company, such other offices in the Company to which he may be
              appointed or assigned from time to time by the Board of Directors
              and/or the Chief Executive Officer of the Company and will
              discharge such duties in connection therewith. The Employee shall
              devote all of his business time to the performance of his duties
              hereunder.

       3.     Compensation. The Company will, during the Term of Employment, pay
              to the Employee as compensation for the performance of his duties
              and obligations hereunder an initial base salary at the rate of
              $200,000 per annum ("Salary"), payable in equal semi-monthly
              installments. Such Salary shall be reviewed annually by the Board
              of Directors of the Company in
<PAGE>   2
Employment Agreement                                                      Page 2

              accordance with the Company's compensation program. In each of the
              Company's fiscal years during the Term of Employment, the Employee
              shall be eligible to receive a bonus, to be awarded at the sole
              discretion of the Board of Directors of the Company, in an amount
              of up to $25,000 per year for the term of the employment
              agreement. The Board shall use as a basis for determining the
              extent of such bonus awards the attainment of stated goals and
              objectives for the Employee to be set by the Compensation
              Committee of the Board after consultation with the Chief Executive
              Officer.

       4.     Other Benefits. During the Term of Employment:

              A.     The Company shall grant to the Employee an incentive stock
                     option, pursuant to the Company's 1995 Stock Option Plan,
                     to purchase 250,000 shares of Common Stock of the Company
                     ("Common Stock") with an exercise price equal to the
                     closing price of the Company's Common Stock on March 14,
                     2000. Such option shall be in the form of and on such terms
                     and conditions as provided in, the Company's standard form
                     of Stock Option Grant Agreement in effect as of the date of
                     this Agreement. Such option grant shall provide, on
                     condition that the Employee is employed by the Company on
                     the relevant vesting dates, that such options shall vest as
                     follows:

                     (1)    10,000 shares shall vest on hire; and

                     (2)    25,000 shares shall vest on the first anniversary of
                            the date of this Agreement and 75,000 shares shall
                            vest over a thirty-six month period at the rate of
                            2,083 shares on the first day of each calendar month
                            commencing on March 14; and

                     (3)    140,000 shares shall vest upon the accomplishment by
                            the Employee of specified milestones, as determined
                            by the Compensation Committee of the Board after
                            consultation with the Chief Executive Officer; and

              B.     The Employee shall be entitled during the Term of
                     Employment to participate in employee benefit plans and
                     programs of the Company to the extent that his position,
                     tenure, salary, age, health and other qualifications make
                     him eligible to participate. The Company does not guarantee
                     the adoption or continuance of any particular employee
                     benefit plan or program during the Term of Employment, and
                     the Employee's participation in any such plan or program
                     shall be subject to the provisions, rules, regulations and
                     laws applicable thereto; provided, however, that during the
                     Term of Employment the Employee shall be entitled to health
                     and hospital insurance benefits consistent with the past
<PAGE>   3
Employment Agreement                                                      Page 3

                     practices of the Company in effect with respect to Company
                     personnel generally; and, further provided, the Employee is
                     eligible for protection under the Alteon Inc. Change in
                     Control Severance Benefits Plan, as in effect from time to
                     time.

              C.     The Employee shall be entitled to three (3) weeks' vacation
                     per year while employed hereunder. Such vacation may be
                     taken by the Employee at such times as do not unreasonably
                     interfere with the business of the Company. The
                     accumulation of annual vacation time earned but not taken
                     will be in accordance with the Company policy guidelines.
                     Additional vacation will be earned in accordance with
                     Company policy.

       5.     Expenses. During the Term of Employment, the Company will
              reimburse the Employee for commuting and housing expenses in an
              amount of $2,500 per month up to a maximum of $30,000 per year. In
              addition, during the Term of Employment, all travel and other
              reasonable business expenses incident to the rendering of services
              by the Employee under this Agreement will be paid or reimbursed by
              the Company subject to the submission of appropriate vouchers and
              receipts in accordance with the Company's policy from time to time
              in effect.

       6.     Death or Disability.

              A.     This Agreement shall be terminated by the death of the
                     Employee. In addition, this Agreement may be terminated by
                     the Board of Directors of the Company if the Employee shall
                     be rendered incapable by illness or any other disability
                     from complying with the terms, conditions and provisions on
                     his part to be kept, observed and performed for a period in
                     excess of 180 days (whether or not consecutive) or 90 days
                     consecutively, as the case may be, during a 12-month period
                     during the Term of Employment ("Disability"). If this
                     Agreement is terminated by reason of Disability of the
                     Employee, the Company shall give written notice to that
                     effect to the Employee in the manner provided herein. In
                     the event that the Employee receives disability insurance
                     benefits paid for by the Company during any period prior to
                     termination of this Agreement pursuant to this Section
                     6(a), the Employee's Salary shall be reduced by an amount
                     equal to such disability insurance benefits during such
                     period.

              B.     In addition to and not in substitution for any other
                     benefits which may be payable by the Company in respect of
                     the death or Disability of the Employee, in the event of
                     such death or Disability, the Salary payable hereunder
                     shall continue to be paid at the then current rate for
                     three (3) months after the termination of employment, and
                     any bonus to which the Employee would have been entitled
                     for the year in which his death occurs
<PAGE>   4
Employment Agreement                                                      Page 4

                     shall be pro rated to the date of his death and paid not
                     later than three (3) months after the termination of
                     employment. In the event of the death of the Employee
                     during the Term of this Agreement, the sums payable
                     hereunder shall be paid to his personal representative.

       7.     Disclosure of Information, Inventions and Discoveries. The
              Employee shall promptly disclose to the Company all processes,
              trademarks, inventions, improvements discoveries and other
              information related to the business of the Company (collectively,
              "Developments") conceived, developed or acquired by him alone or
              with others during the Term of Employment or during any earlier
              period of employment by the Company or any predecessor of the
              Company, whether or not during regular working hours or through
              the use of materials or facilities of the Company. All such
              Developments shall be the sole and exclusive property of the
              Company, and, upon request, the Employee shall promptly deliver to
              the Company all drawings, sketches, models and other data and
              records relating to such Developments. In the event any such
              Development shall be deemed by the Company to be patentable, the
              Employee shall, at the expense of the Company, assist the Company
              in obtaining a patent or patents thereon and execute all documents
              and do all such other acts and things necessary or proper to
              obtain letters of patents and to invest in the Company full right,
              title and interest in and to such Developments.

       8.     Non-Disclosure. The Employee shall not, at any time during or
              after the Term of Employment, divulge, furnish or make accessible
              to anyone (otherwise than in the regular course of business of the
              Company) or use for his own account or for the account of any
              person any knowledge or information with respect to confidential
              or secret processes, inventions, discoveries, improvements,
              formulae, plans, materials, devices or ideas or other know-how,
              whether patentable or not, with respect to any confidential or
              secret development or research work or with respect to any other
              confidential or secret aspects of the Company's business
              (including, without limitation, customer lists, supplier lists and
              pricing arrangements with customers or suppliers).

       9.     Non-Competition. The Company and the Employee agree that the
              services rendered by the Employee hereunder are unique and
              irreplaceable. The Employee hereby agrees that, during the Term of
              Employment and for a period of one (1) year thereafter, the
              Employee shall not (i) in any geographical area in the United
              States or in those foreign countries where the Company, during the
              Term of Employment, conducts or proposes to conduct business or
              initiates activities, engage or participate in, directly or
              indirectly (whether as an officer, director, employee, partner,
              consultant, holder of an equity or debt investment, lender or in
              any other manner or capacity), or lend
<PAGE>   5
Employment Agreement                                                      Page 5

              his name (or any part or variant thereof) to any business which
              is, or as a result of the Employee's engagement or participation
              would become, competitive with any aspect of the business of the
              Company, such business being the commercialization of the
              measurement, prevention therapy or reversal of glucose-mediated
              non-enzymatic cross-linking of macro-molecules, and such other
              specific technologies in which the Company has, during the Term of
              Employment, initiated significant plans to develop products, (ii)
              deal, directly or indirectly, in a competitive manner with any
              customers doing business with the Company during the Term of
              Employment (except in connection with the performance of the
              duties and obligations of the Employee during the Term of
              Employment), (iii) solicit any officer, director, employee,
              consultant or agent of the Company to become an officer, director,
              employee, consultant or agent of the Employee, his respective
              affiliates or anyone else, and (iv) engage in or participate in,
              directly or indirectly, any business conducted under any name that
              shall be the same as or similar to the name of the Company or any
              trade name used by it. Ownership, in the aggregate, of less than
              1% of the outstanding shares of capital stock of any corporation
              with one or more classes of its capital stock listed on a national
              securities exchange or publicly traded in the over-the-counter
              market shall not constitute a violation of the foregoing
              provision.

       10.    Remedies. The Employee acknowledges that irreparable damage would
              result to the Company if the provisions of Section 7, 8, 9 or 14
              were not specifically enforced, and agrees that the Company shall
              be entitled to any appropriate legal, equitable or other remedy,
              including injunctive relief, in respect to any failure to comply
              with the provisions of Section 7, 8, 9 or 14.

       11.    Termination for Cause. In addition to any other remedy available
              to the Company, either at law or in equity, the Employee's
              employment with the Company may be terminated by the Board of
              Directors for cause, which shall include (i) the Employee's
              conviction for, or plea of nolo contendere to, a felony or a crime
              involving moral turpitude, (ii) the Employee's commission of an
              act of personal dishonesty or a breach of fiduciary duty involving
              personal profit in connection with the Employee's employment by
              the Company, (iii) the Employee's commission of an act which the
              Board of Directors shall reasonably have found to have involved
              willful misconduct or gross negligence on the part of the
              Employee, in the conduct of his duties under this Agreement, (iv)
              habitual absenteeism, (v) the Employee's material breach of any
              material provision of this Agreement, or (vii) the willful and
              continued failure by the Employee to perform substantially his
              duties with the Company (other than any such failure resulting
              from his incapacity due to physical or mental illness). In the
              event of termination under this Section 11, the Company's
              obligations under this Agreement shall cease and the Employee
              shall forfeit all rights to receive any future compensation under
              this
<PAGE>   6
Employment Agreement                                                      Page 6

              Agreement. Notwithstanding any termination of this Agreement
              pursuant to this Section 11, the Employee, in consideration of his
              employment hereunder to the date of such termination, shall remain
              bound by the provisions of Section 7, 8, 9 and 14 hereof.

       12.    Termination Without Cause. Each of the Company and Employee may
              terminate this Agreement at any time for any reasons whatsoever,
              without any further liability or obligation of the Company to the
              Employee or of the Employee to the Company from and after the date
              of such termination (other than liabilities or obligations accrued
              but unsatisfied on, or surviving, the date of such termination),
              by sending thirty (30) days prior written notice to the other
              party. In the event (a) the Company elects to terminate this
              Agreement prior to the end of the Term of Employment or (b) the
              Company gives Employee notice of its election not to extend the
              Term of Employment beyond the expiration of the then current Term
              of Employment, or (c) by the date which is four (4) months prior
              to the end of the then current Term of Employment, the Company has
              not offered to extend the then current Term of Employment, the
              Company shall continue to pay the Employee the full Salary
              (exclusive of bonuses, if any) as such Salary would have otherwise
              accrued for a period of three (3) months if the effective date of
              such termination occurs prior to the first anniversary of this
              Agreement and for a period of six (6) months if the effective date
              of such termination occurs thereafter. In the event the Employee
              elects to terminate prior to the end of the Term of Employment,
              the Company's obligation to pay Salary shall cease as of the
              effective date of termination. Notwithstanding any termination of
              this Agreement pursuant to this Section 12, the Employee, in
              consideration of his employment hereunder to the date of such
              termination, shall remain bound by the provisions of Section 7, 8,
              9 and 14 hereof. Any termination of this Agreement by the Company
              as provided in this Section 12 shall be in addition to, and not in
              substitution for, any rights with respect to termination of the
              Employee which the Company may have pursuant to Section 11.

       13.    Resignation. In the event that the Employee's services under this
              Agreement are terminated under any of the provisions of this
              Agreement (except by death), the Employee agrees that he will
              deliver his written resignation from all positions held with the
              Company to the Board of Directors, such resignation to become
              effective immediately; provided, however, that nothing herein
              shall be deemed to affect the provisions of Section 7, 8, 9 and 14
              hereof relating to the survival thereof following termination of
              the Employee's services hereunder, and provided, further, that
              except as expressly provided in this Agreement, the Employee shall
              be entitled to no further compensation hereunder.
<PAGE>   7
Employment Agreement                                                      Page 7

       14.    Data. Upon termination of the Term of Employment or termination
              pursuant to Sections 6, 11 or 12 hereof, the Employee or his
              personal representative shall promptly deliver to the Company all
              books, memoranda, plans, records and written data of every kind
              relating to the business and affairs of the Company which are then
              in his possession.

       15.    Insurance. The Company shall have the right, at its own cost and
              expense to apply for and to secure in its own name, or otherwise,
              life, health or accident insurance or any or all of them covering
              the Employee, and the Employee agrees to submit to usual and
              customary medical examinations and otherwise to cooperate with the
              Company in connection with the procurement of any such insurance,
              and any claims thereunder.

       16.    Waiver of Breach. Any waiver of any breach of this Agreement shall
              not be construed to be a continuing waiver or consent to any
              subsequent breach on the part either of the Employee or of the
              Company.

       17.    Assignment. This Agreement shall inure to the benefit of and be
              binding upon the successors and assigns of the Company upon any
              sale of all or substantially all of the Company's assets, or upon
              any merger or consolidation of the Company with or into any other
              entity, all as though such successors and assigns of the Company
              and their respective successors and assigns were the Company.
              Insofar as the Employee is concerned, this Agreement, being
              personal, may not be assigned.

       18.    Severability. To the extent any provision of this Agreement shall
              be invalid or unenforceable, it shall be considered deleted
              therefrom and the remainder of such provision and of this
              Agreement shall be unaffected and shall continue in full force and
              effect. In furtherance and not in limitation of the foregoing,
              should the duration or geographical extent of, or business
              activities covered by, any provision of this Agreement be in
              excess of that which is valid and enforceable under applicable
              law, then such provision shall be construed to cover only that
              duration, extent or activities which may be validly and
              enforceable covered.

       19.    Notices. All notices, requests and other communications pursuant
              to this Agreement shall be in writing and shall be deemed to have
              been duly given, if delivered in person or by courier,
              telegraphed, telexed or by facsimile transmission or five business
              days after being sent by registered or certified mail, return
              receipt requested, postage paid, addressed as follows:
<PAGE>   8
Employment Agreement                                                      Page 8

              If to the Employee

                           Robert deGroof, Ph.D.
                           145 Water Crest Drive
                           Doylestown, Pennsylvania 18901

              If to the Company:

                           Alteon Inc.
                           170 Williams Drive
                           Ramsey, NJ 07446

              with a copy to:

                           Richard J. Pinto, Esq.
                           Smith, Stratton, Wise, Heher & Brennan
                           600 College Road East
                           Princeton, NJ 08540

              Any party may, by written notice to the other in accordance with
              this Section 19, change the address to which notices to such party
              are to be delivered or mailed.

       20.    General. Except as otherwise provided herein, the terms and
              provisions of this Agreement and the Stock Option Grant Agreement
              to be entered into between the Employee and the Company shall
              constitute the entire agreement by the Company and the Employee
              with respect to the subject matter hereof, and shall supersede any
              and all prior agreements or understandings between the Employee
              and the Company, whether written or oral. This Agreement may be
              amended or modified only by a written instrument executed by the
              Employee and the Company. This Agreement may be executed in any
              number of counterparts, all of which, when executed, shall be
              deemed to be an original, and all of which together shall
              constitute one and the same instrument.
<PAGE>   9
Employment Agreement                                                      Page 9

       IN WITNESS WHEREOF, the parties have executed this Employment Agreement
as of the day and year first above written.

                                              ALTEON INC.

                                              By:   /s/ Kenneth I. Moch
                                                 ------------------------------
                                                    Kenneth I. Moch
                                                    Chief Executive Officer

                                                    /s/ Robert deGroof, Ph.D.
                                                 ------------------------------
                                                    Robert deGroof, Ph.D.

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