<PAGE>

                                                                   EXHIBIT 10.56

                          FIFTEENTH AMENDMENT TO LEASE

      This Amendment made as of the 16th day of June, 1999 is by and between
GROVE STREET ASSOCIATES OF JERSEY CITY LIMITED PARTNERSHIP, a New Jersey limited
partnership having an office at c/o Mack-Cali Realty Corporation, 11 Commerce
Drive, Cranford, New Jersey 07016 (hereinafter called "Landlord"), and
DONALDSON, LUFKIN & JENRETTE SECURITIES CORPORATION, a Delaware corporation,
having an office at 277 Park Avenue, New York, New York 10172 (hereinafter
called "Tenant").

                              W I T N E S S E T H

      WHEREAS, Landlord and Tenant have previously entered into a Lease as
described on the attached Schedule #1 (collectively, the "Lease") covering
certain premises in the building at 1 Pershing Plaza, Jersey City, New Jersey
(the "Building"); and

      WHEREAS, Tenant wishes to lease the space on floor 12A of the Building
currently leased to Equitable Life Assurance Society of the United States.

      WHEREAS, the Expiration Date of the Lease is July 13, 2009.

      WHEREAS, Landlord and Tenant desire to amend the Lease as set forth
herein.

      NOW, THEREFORE, in consideration of the foregoing, the sum of Ten Dollars
($10.00) and other good and valuable consideration the receipt and sufficiency
of which are hereby acknowledged, Landlord and Tenant do hereby agree to amend
and modify the Lease as follows:

1.    All defined terms used in this Amendment shall have the meanings ascribed
      to them in the Lease unless otherwise defined herein.

2.    The recitals set forth above are incorporated herein by reference.

3.    From and after July 1, 1999, Landlord hereby leases to Tenant and Tenant
      hereby hires from Landlord approximately 6,019 rentable square feet on
      floor 12A of the Building (the "Equitable Space") as shown cross-hatched
      on the attached Exhibit A. Tenant has fully inspected the Equitable Space,
      is satisfied with the condition thereof, and agrees to accept possession
      of the Equitable Space in its current "AS-IS" condition, subject to
      reasonable wear and tear and damage by fire or other cause.

4.    The "Equitable Space Possession Date" shall be June 1,1999. For the period
      commencing on the Equitable Space Possession Date and ending on the date
      immediately proceeding the Equitable Space Commencement Date (the
      "Construction Period"), Tenant shall be entitled to occupy the Equitable
      Space for the purpose of performing improvements to the Equitable Space
      pursuant to Article 13 of the Lease. Tenant's occupancy of the Equitable
      Space during the Construction Period shall be subject to all of the terms
      and conditions of the Lease (including without limitation, Tenant's
      indemnification obligation pursuant to Article 21 of the Lease and
      Tenant's obligation to maintain insurance pursuant to Article 11 of the
      Lease.)

5.    The term applicable to the Equitable Space shall commence on July 1, 1999
      (the "Equitable Commencement Date") and shall expire on the Expiration
      Date of the Lease.

6.    The first sentence of Section 1.02 of the Lease is hereby amended to read
      in its entirety as follows:

      "The premises hereby leased to Tenant are described on the attached
      Schedule #2."

7.    In addition to the fixed rent to be paid to Landlord by Tenant for the
      Premises as previously set forth in the Lease, Tenant shall pay Landlord
      fixed rent for the Equitable Space as follows:

      (i)   THREE THOUSAND SEVEN HUNDRED FOUR AND 59/100 DOLLARS ($3,704.59) for
            the period from July 1, 1999 to July 13, 1999;

      (ii)  ELEVEN THOUSAND FOUR HUNDRED EIGHTY-SIX AND 26/100 DOLLARS
            ($11,486.26) per month for the period commencing July 14, 1999 to
            July 13, 2004

                                       1
<PAGE>

            and

      (iii) THIRTEEN THOUSAND SEVEN HUNDRED EIGHTY-THREE AND 51/100 DOLLARS
            ($13,783.51) per month for the period commencing July 14, 2004 to
            July 13, 2009.

      The fixed rent shall be payable in the same manner and under the same
      terms and conditions as the fixed rent currently provided in the Lease.

8.    As of July 1, 1999, Section 5.01(e) of the Lease is amended to provide
      that Tenant's Proportionate Share shall be 69.37% (68.41% + .96%), and
      that the agreed upon rentable square foot area of the Demised Premises
      shall be deemed to be 431,491 square feet. As of October 1, 2000, Section
      5.01(e) of the Lease is amended to provide that Tenant's Proportionate
      Share shall be 73.29% (69.37% + 3.92%), and that the agreed upon rentable
      square foot area of the Demised Premises shall be deemed to be 455,870
      square feet.

9.    As of July 1, 1999, Section 5.07(k) of the Lease is amended to provide
      that Tenant's Operational Proportionate Share shall be 70.13% (69.16% +
      .97%). As of October 1, 2000, Section 5.07(k) of the Lease is amended to
      provide that Tenant's Operational Proportionate Share shall be 74.09%
      (70.13% + 3.96%).

10.   The garage parking space allocation applicable to the Equitable Space is
      four (4) spaces. At any time during the Term, Tenant may elect to lease
      all or less than all of these spaces by sending written notice to Landlord
      of its intention to lease same. Upon Tenant's election to lease the garage
      parking spaces, Tenant agrees to pay Landlord the monthly rate Landlord is
      then charging other tenants in the Building for each space so leased.

11.   a.    The cost of electric current which is supplied by Landlord for
            use by Tenant in the Equitable Space, other than for air
            conditioning purposes, shall be reimbursed to the Landlord at terms,
            classification and rates normally charged by the public utilities
            corporation serving that part of the municipality where the Premises
            are located.

      b.    Landlord shall, prior to July 1, 1999, estimate the electric power
            demand of the electric lighting fixtures and the electric equipment
            of Tenant to be used in the Equitable Space to determine the average
            monthly electric consumption thereof. During the Term, Tenant shall
            pay to Landlord, in advance, on the first day of every month in
            conjunction with Tenant's monthly payments of fixed rent, the amount
            estimated by Landlord as Tenant's monthly consumption. Said amounts
            shall be treated as Additional Rent due hereunder. Proportionate
            sums shall be payable for periods of less than a full month if the
            term commences or ends on any other than the first or last day of
            the month. Within sixty (60) days of July 1,1999, Tenant agrees that
            Landlord's electrical engineering consultant shall make a survey of
            electric power demand of the electric lighting fixtures and the
            electric equipment of Tenant used in the Equitable Space to
            determine the average monthly electric consumption thereof, and the
            costs of said survey shall be borne by Tenant. The findings of said
            consultant as to the average monthly electric consumption of Tenant
            shall, unless objected to by Tenant within thirty (30) days, be
            conclusive and binding on Landlord and Tenant. After Landlord's
            consultant has submitted its report, Tenant shall pay to Landlord,
            within ten (10) days after demand therefor by Landlord, any
            underpayment (based on the monthly consumption found by such
            consultant as compared to Landlord's estimate) as owing from the
            Equitable Commencement Date, and the then expired months, to include
            the then current month and thereafter, on the first day of every
            month, in advance, the amount set forth as the monthly consumption
            in said report. Any overpayments made by Tenant shall be credited
            against the next electrical charges then payable to Landlord. If
            Tenant objects to Landlord's consultant's survey, Tenant shall
            nevertheless pay and continue to pay the amount determined by
            Landlord's consultant until the issue is finally resolved, but
            Tenant may, at its expense, seek the services of an independent
            electrical consultant who shall make a survey as provided above. If
            Landlord and Tenant's consultant cannot agree as to Tenant's
            consumption within thirty (30) days of Tenant's consultant's
            findings, either Landlord or Tenant may request the American
            Arbitration Association in Somerset, New Jersey to appoint an
            electrical engineering consultant whose decision shall be final and
            binding on Landlord and Tenant, and whose cost shall be shared
            equally. Upon the issue being finally resolved, any overpayment made
            by Tenant shall promptly be refunded by Landlord and any


                                        2
<PAGE>


            underpayment in Tenant's payments shall promptly be paid to
            Landlord.

12.   On or before August 1, 1999, Landlord shall provide Tenant a check in the
      amount of EIGHTY-ONE THOUSAND, EIGHT HUNDRED EIGHTY-THREE AND 96/100
      ($81,883.96) towards the cost of improving the Premises (the "Equitable
      Improvement Allowance"). The Equitable Improvement Allowance must be used
      by June 30, 2000, or the allowance shall be withdrawn. All work performed
      in the Equitable Space be governed by the provisions of Article 13
      "TENANT'S CHANGES" of the Lease. At Landlord's request, Tenant shall
      furnish Landlord with copies of paid invoices evidencing the work
      performed in the Premises which equals or exceeds the Equitable
      Improvement Allowance.

13.   With respect to this Fifteenth Amendment to Lease. Article 32 (Broker) is
      hereby deleted and the following shall apply to the leasing of the
      Equitable Space: "Landlord and Tenant represent and warrant to each other
      that no broker brought about this transaction, and the parties agree to
      indemnify and hold each other harmless from any and all claims of any
      broker (claiming to have dealt with the indemnifying party) arising out of
      or in connection with the negotiations of or entering into of this
      Amendment by Tenant and Landlord. Landlord represents that there is no
      commission due to Cushman and Wakefield in connection with this
      transaction pursuant to any prior agreement between Landlord and Cushman
      and Wakefield.

14.   Tenant represents and warrants that this Amendment and the undersigned's
      execution of same has been duly authorized and approved by the
      corporation's Board of Directors. The undersigned officer of the
      corporation represents and warrants he is an officer of the corporation
      with authority to execute this Amendment on behalf of the corporation.
      Landlord represents and warrants that this Amendment and the undersigned's
      execution of same has been duly authorized and approved by all necessary
      partnership and corporate actions. The undersigned officer of the
      corporate general partner of Landlord represents and warrants that he is
      an officer of the corporation with authority to execute this Amendment on
      behalf of the corporate general partner of Landlord, and that the
      corporate general partner of Landlord is authorized to execute this
      Amendment on behalf of Landlord.

15.   EXCEPT as modified herein, the Lease dated July 1, 1987 and all
      amendments and sideletters applicable thereto covering the Premises shall
      remain in full force and effect as if the same had been set forth in full
      herein and Tenant and Landlord hereby ratify and confirm all of the terms
      and conditions thereof. Tenant acknowledges that it has no offsets,
      defenses or counterclaims to its obligations under the Lease as amended
      hereby.

      THIS Amendment shall be binding upon and inure to the benefit of the
parties hereto and their respective legal representatives, successors and
permitted assigns.


GROVE STREET ASSOCIATES OF                DONALDSON, LUFKIN & JENRETTE
JERSEY CITY LIMITED PARTNERSHIP,          SECURITIES CORPORATION,
LANDLORD                                  TENANT

By:   Mack-Cali Sub IV, Inc.
      General Partner


By:   /s/ James G. Nugent                 By:   /s/ Robert A. Yurman
      -----------------------------             --------------------------------
      James G. Nugent                           Robert A. Yurman, Sr. Vice
      Sr. Vice President - Leasing              President
                                                Director of Administration



                                       3
<PAGE>



                                    EXHIBIT A
                           LOCATION OF EQUITABLE SPACE

                                    [GRAPHIC]




<PAGE>

                     SCHEDULE #1 TO LEASE DATED JULY 1, 1987

      LANDLORD: Grove Street Associates of Jersey City Limited Partnership

      TENANT: Donaldson, Lufkin & Jenrette Securities Corporation


                            DESCRIPTION OF THE LEASE

<TABLE>
<CAPTION>

           DATE OF DOCUMENT        DOCUMENT                          SUBJECT
           ----------------        --------                          -------

<S>                                <C>                               <C>
           July 1,1987             Lease                             Original Document
           July 1, 1987            First Amendment of the Lease      10th Floor Space
           July 1, 1987            Side Letter Amendment
           June 19, 1989           Side Letter Agreement
           March 12, 1992          Second Amendment to Lease         Mezz. B - Storage Space
           December 27, 1992       Third Amendment to Lease          11th Floor Space
           September 29, 1993      Side Letter Agreement
           December 23, 1993       Fourth Amendment to Lease         Mezz. B - Storage Space
           May 1, 1994             Fifth Amendment to Lease          12th Floor - Temp. Space
           March 9, 1995           Sixth Amendment to Lease          Mezz. B - Temp. Storage Space
           June 16, 1995           Seventh Amendment to Lease        14th Flr. - Temp. Storage Space
           April 4, 1996           Eighth Amendment to Lease         14th & 15th Floor Space
           April 4, 1996           Ninth Amendment to Lease          14th & 15th Floor Space
           December 31, 1996       Tenth Amendment to Lease          Floor 12A Space
           February 7, 1997        Eleventh Amendment to Lease       Dry cooler installation
           August 18, 1997         Twelfth Amendment to Lease        12th Floor Space - 4,809 sf
           January 12, 1998        Thirteenth Amendment to Lease     Strachan Space (Floors 12 & 12A)
           December 28, 1998       Fourteenth Amendment to Lease     CDR Space (12th Floor Space)
</TABLE>












<PAGE>

                     SCHEDULE #2 TO LEASE DATED JULY 1, 1987

      LANDLORD: Grove Street Associates of Jersey City Limited Partnership

           TENANT: Donaldson, Lufkin & Jenrette Securities Corporation


                       DESCRIPTION OF THE LEASED PREMISES


<TABLE>
<CAPTION>

                                     OFFICE          STORAGE        TEMPORARY
    FLOOR(S)                         SPACE           SPACE            SPACE
    --------                         -----           -----            -----
<S>                                 <C>               <C>                 <C>

               2-9                  256,926
               10                    35,820
                                 ----------
              2-10                  292,745
              Lobby                   4,800
                                 ----------
  Lobby, 2-10                       297,546
               11                    36,600
                                 ----------
  Lobby, 2-11                       334,145
  Storage (kitchen)                                    8,194
  Storage Space #1                                     1,800
  Storage Space #2                                     3,747
  Storage-Temporary #1                                                    794
             14&15                   73,200
             12A                      6,507
             12                       4,809
                                 ----------
  Subtotal                          418,661           13,741              794
   CDR Space                          6,811                0                0
                                 ----------
  Subtotal                          425,472           13,741              794
   Equitable Space                    6,019                0                0
                                 ----------          -------              ---
  Subtotal                          431,491           13,741              794
   Strachan 12                       10,307                0                0
   Strachan 12A                      14,072                0                0
                                 ----------
 TOTAL S.F. AREAS                   456,870           13,741              794
                                 ----------          -------              ---
</TABLE>