<PAGE>

                                                                   EXHIBIT 10.1


                            NINTH AMENDMENT TO LEASE
                            ------------------------


     NINTH AMENDMENT TO LEASE dated as of this 24th day of February, 2000, by
and between BP PRUCENTER ACQUISITION, LLC, a Delaware limited liability
company (as successor-in-interest to The Prudential Insurance Company of
America) ("Landlord") and DIGITAS LLC, formerly known as BRONNERCOM, LLC,
formerly known as BRONNER SLOSBERG HUMPHREY, LLC, a Delaware limited
liability company, as successor-in-interest to Bronner Slosberg Humphrey,
Inc. ("Tenant").

                                  R E C I T A L S
                                  ---------------

     By Lease dated May 31, 1995, as amended by a First Amendment of Lease
dated June 21, 1996, a Second Amendment of Lease dated September 1, 1996, a
Third Amendment of Lease dated November 5, 1996, a Fourth Amendment of Lease
dated January 22, 1997, a Fifth Amendment and Partial Termination of Lease
dated July 11, 1997, a Sixth Amendment and Partial Termination of Lease dated
May 15, 1998, a Seventh Amendment to Lease dated March 29, 1999 and an Eighth
Amendment to Lease dated July 30, 1999 (the "Lease"), Landlord's
predecessor-in-interest did lease to Tenant and Tenant did hire and lease
from Landlord's predecessor-in-interest certain premises (the "Initial
Premises") located in the Tower Building (the "Building"), Prudential Center,
Boston, Massachusetts, which Initial Premises are described with greater
particularity in the Lease.

     Tenant has exercised its right, pursuant to Paragraph C of the Eighth
Amendment to Lease, to lease the First Offer Space, being the balance of the
fifth (5th) floor of the Building, substantially as shown on Exhibit A to the
Eighth Amendment to Lease.

     Landlord and Tenant are entering into this instrument to set forth said
leasing of the Fourth Floor Substitution Space, the Ninth Amendment Space and
the First Offer Space into the Lease and to amend the Lease.

     NOW THEREFORE, in consideration of One Dollar ($1.00) and other good and
valuable consideration in hand this date paid by each of the parties to the
other, the receipt and sufficiency of which are hereby severally
acknowledged, and in further consideration of the mutual promises herein
contained, Landlord and Tenant hereby agree to and with each other as follows:

     A.  EIGHTH AMENDMENT SPACE B

         Reference is made to the Eighth Amendment Space B demised by Tenant
         pursuant to Paragraph B of the Eighth Amendment to Lease.
         Notwithstanding anything to the contrary contained in the Eighth
         Amendment to Lease, the parties desire to revise the Eighth
         Amendment Space B Commencement Date.

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         Therefore, Paragraph B(2) of said Eighth Amendment is hereby deleted
         and the following is hereby substituted in its place:

             "The Eighth Amendment Space B Commencement Date shall be the
             day after the date that the present tenant of the Eighth
             Amendment Space B vacates the Eighth Amendment Space B. The
             parties hereby acknowledge that the present tenant of the
             Eighth Amendment Space B is currently holding over in the
             Eighth Amendment Space B after the expiration of the term of
             its lease on January 31, 2000. Landlord hereby represents that
             Landlord is diligently pursuing its rights and remedies
             against such tenant to obtain possession of the Eighth
             Amendment Space B. Therefore, Landlord currently estimates
             that the Eighth Amendment Space B Commencement Date will be on
             or about March 15, 2000. Tenant shall lease the Eighth
             Amendment Space B in "as-is" condition, broom clean and clear
             of all occupants, in the condition in which the Eighth
             Amendment Space B is in as of the Eighth Amendment Space B
             Commencement Date, without any obligation on the part of
             Landlord to prepare or construct the Eighth Amendment Space B
             for Tenant's occupancy, expect that Landlord shall provide to
             Tenant an allowance of $5.00 per square foot of rentable
             floor area of the Eighth Amendment Space B (the "Eighth
             Amendment Space B Allowance") towards the cost of improvements
             to be performed by Tenant on the fifth (5th) floor in
             accordance with the terms of the Lease after Tenant delivers
             to Landlord paid invoices indicating the actual cost of such
             improvements reasonably satisfactory to Landlord.
             Notwithstanding the foregoing, Landlord shall be under no
             obligation to apply any portion of the Eighth Amendment Space
             B Allowance for any purposes other than as provided in this
             Ninth Amendment, nor shall Landlord be deemed to have assumed
             any obligations, in whole or in part, of Tenant to any
             contractors, subcontractors, suppliers, workmen or
             materialmen. In addition, Landlord shall not be obligated to
             make any application of any portion of the Eighth Amendment
             Space B Allowance if (i) there shall be existing any default
             of Tenant under the Lease (as defined in Section 8.1), beyond
             the expiration of applicable notice and grace periods, or (ii)
             there are any liens which are not bonded to the reasonable
             satisfaction of the Landlord against Tenant's interest in the
             Lease or against the Premises, the Building, or the Prudential
             Center arising out of any work performed pursuant to the Lease
             by Tenant or any litigation in which Tenant is a party."


                                     -2-

<PAGE>


     B.  FIRST OFFER SPACE

         1. Reference is made to the fact that Tenant has exercised its right,
            pursuant to Paragraph C of the Eighth Amendment to Lease, to lease
            the First Offer Space. Reference is further made to the fact that,
            as a result of such exercise by Tenant, Tenant has demised the
            entirety of the fifth (5th) floor of the Building and that those
            portions of the fifth (5th) floor which were previously common
            areas shall now be included as a part of the First Offer Space.
            Therefore, notwithstanding anything to the contrary in said
            Paragraph C of the Eighth Amendment to Lease, the RFO Premises is
            hereby agreed to consist of 5,815 rentable square feet, provided
            however, that for the purposes of determining the rent and
            Tenant's Share in respect of the First Offer Space, the First
            Offer Space shall be deemed to be 3,298 rentable square feet.

         2. Effective as of the "RFO Premises Commencement Date" (as defined
            in Section 3 hereof) the RFO Premises shall constitute a part of
            the "Premises" (as defined and used in the Lease), so that the
            Premises (as defined and used in the Lease) shall include the
            Initial Premises and the RFO Premises.

         3. The RFO Premises Commencement Date shall be February 1, 2000.
            Tenant shall lease the RFO Premises in "as-is" condition,
            broom clean and clear of all occupants, in the condition in which
            the RFO Premises are in as of the RFO Premises Commencement Date,
            without any obligation on the part of Landlord to prepare or
            construct the RFO Premises for Tenant's occupancy and without
            any obligation to provide any allowance to Tenant in respect of
            the RFO Premises, except that Landlord shall, at Landlord's cost,
            perform such work in the women's restroom in the RFO Premises
            as is necessary to bring such restroom into compliance with the
            Americans with Disabilities Act ("Landlord's ADA Work"). Landlord
            shall use reasonable efforts to perform Landlord's ADA Work on
            or before March 31, 2000.

         4. The Term of the Lease for the RFO Premises shall commence on the RFO
            Premises Commencement Date and shall expire on November 30, 2005,
            unless sooner terminated in accordance with the provisions of the
            Lease as herein amended, upon all the same terms and conditions
            contained in the Lease as herein amended.

         5. Tenant's obligation to pay rent, Increased Operating Expenses and
            increased Real Estate Taxes in respect of the RFO Premises shall
            not commence to accrue until the date ("RFO Premises Rent
            Commencement Date") which is the earlier of (x) the Commencement
            Date in respect of Eighth Amendment Space B, as defined in
            Paragraph A of this Ninth Amendment to Lease, or (y) the date that
            Tenant begins to move into the RFO Premises furniture and
            equipment for its regular business operations.


                                     -3-

<PAGE>

         6. Rent for the RFO Premises from the RFO Premises Rent
            Commencement Date through the expiration of the RFO Premises
            Lease Term shall be payable at the annual rate of $148,410.00
            (being the product of (i) $45.00 and (ii) the rentable floor
            area of the RFO Premises (being 3,298 square feet).

         7. Commencing on the RFO Premises Rent Commencement Date, Tenant
            shall reimburse Landlord for Increased Operating Expenses
            (pursuant to Section 3.2 of the Lease) for the RFO Premises
            in excess of the Operating Expenses incurred by Landlord
            during calendar year 1999 (being January 1, 1999 through
            December 31, 1999).

         8. Commencing on the RFO Premises Rent Commencement Date, Tenant
            shall reimburse Landlord for increased Real Estate Taxes
            (pursuant to Section 3.4 of the Lease) for the RFO Premises in
            excess of the Real Estate Taxes paid by Landlord in fiscal year
            2000 (being July 1, 1999 through June 30, 2000).

         9. Tenant's Share with respect to the RFO Premises consisting of
            a total of 3,298 rentable square feet with be .27%.

        10. Tenant shall reimburse Landlord monthly in arrears within thirty
            (30) days of receipt of Landlord's invoice for the cost of
            electricity consumed in the RFO Premises as determined by a
            check meter (which check meter is presently installed) at
            Landlord's cost of electricity from time to time.

        11. Tenant shall not be entitled to any additional parking passes in
            connection with Tenant's demise of the RFO Premises.

        12. Tenant's extension rights set forth in Section 12.15 of the
            Lease, as amended by Section II(1) of the Fourth Amendment of
            Lease, shall apply to the RFO Premises.

        13. There shall be no modification of the terms regarding the
            reduction and/or increase in the Original LC and/or the Third
            Amendment LC in connection with Tenant's demise of the RFO
            Premises.

     C.  OTHER TERMS

         1. Tenant hereby acknowledges and confirms that the indemnity set
            forth in Section 11.1 of the Lease shall apply, and always has
            applied, to the use or occupancy by Tenant of the Premises in
            its entirety and to the acts or omissions of Tenant, its agents,
            employees or any contractors brought onto the Premises in its
            entirety by Tenant.


                                     -4-

<PAGE>
         2. (A) Tenant warrants and represents that Tenant has not dealt
            with any broker in connection with the consummation of this
            Ninth Amendment other than Meredith & Grew, Incorporated
            (the "Recognized Broker"); and in the event any claim is made
            against Landlord relative to dealings by Tenant with brokers
            other than the Recognized Broker, Tenant shall defend the
            claim against Landlord with counsel of Tenant's selection
            first approved by Landlord (which approval will not be
            unreasonably withheld) and save harmless and indemnify
            Landlord on account of loss, cost or damage which may arise
            by reason of such claim.

            (B) Landlord warrants and represents that Landlord has not
            dealt with any broker in connection with the consummation of
            this Ninth Amendment other than the Recognized Broker; and
            in the event any claim is made against Tenant relative to
            dealings by Landlord with brokers other than the Recognized
            Broker, Landlord shall defend the claim against Tenant with
            counsel of Landlord's selection and save harmless and
            indemnify Tenant on account of loss, cost or damage which
            may arise by reason of such claim. Landlord shall be
            responsible for paying the commission due to the Recognized
            Broker in connection with this Ninth Amendment.

         3. Except as otherwise expressly provided herein, all capitalized
            terms used herein without definition shall have the same
            meanings as are set forth in the Lease.

         4. Except as herein amended the Lease shall remain unchanged and in
            full force and effect. All references to the "Lease" shall be
            deemed to be references to the Lease as herein amended.


                                     -5-

<PAGE>


     EXECUTED as a sealed instrument as of the date and year first above
written.

WITNESS:                               LANDLORD:

/s/ Susan J. Murphy                    BP PRUCENTER ACQUISITION, LLC
    --------------
                                       By: BOSTON PROPERTIES LIMITED
                                           PARTNERSHIP, its Manager

                                           By: BOSTON PROPERTIES, INC.,
                                               its general partner

                                               By    /s/ Claude B. Hoopes
                                                     -----------------------
                                               Name    Claude B. Hoopes
                                                     -----------------------
                                               Title   Senior Vice President
                                                     -----------------------


WITNESS:                               TENANT:


    ------------
                                       DIGITAS LLC

                                               By    /s/ Marschall I. Smith
                                                     -----------------------
                                               Name    Marschall I. Smith
                                                     -----------------------
                                               Title   Secretary and
                                                       General Counsel
                                                     -----------------------





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