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                                                                  EXHIBIT 10.M.I

                           FOURTH AMENDMENT TO LEASE

        This FOURTH AMENDMENT TO LEASE ("Amendment") entered into as of
September 1, 1999, by and between FRANCIS J. PERRY, JR., FRANCIS J. PERRY III,
AND WILLIAM J. WALKER, JR. AS TRUSTEES OF ROSEMONT TRUST ("Trust") under
declaration of trust dated November 19, 1980, recorded with the NorfolK County
Registry of Deeds ("Registry") in Book 5856, Page 213, as amended by First
Amendment to Rosemont Trust dated December 22, 1986, recorded with Norfolk Deeds
in Book 7528, Page 525 and by Second Amendment to Declaration of Trust
Establishing Rosemont Trust dated March 31, 1995, recorded with Norfolk Deeds in
Book 11012, Page 206 ("Landlord"), as Landlord, and LTX CORPORATION, a
Massachusetts corporation ("Tenant"), as Tenant.

        REFERENCES AND RECITALS
        -----------------------

        Reference is made to the Third Amendment to Lease and Restatement of
Lease ("Lease") entered into April 29, 1982, by and between Landlord's
predecessors as trustees of the Trust as landlord and Tenant as Tenant. The
Lease amended, and restated in its entirety, a lease ("1980 Lease") originally
executed November 26, 1980 by and between Landlord's predecessors as trustees of
the Trust as Landlord and Tenant as Tenant, as the 1980 Lease had been
previously amended. Notice of the 1980 Lease was recorded with the Registry in
Book 5856, Page 228. Notice of the Lease was provided by a Second Amendment to
Notice of Lease recorded with the Registry in Book 5996, Page 564. All terms not
specifically defined in this Amendment have the meanings given to them in the
Lease.

        Landlord and Tenant now wish to amend the Lease in certain respects as
set forth below.

        FOR GOOD AND VALUABLE CONSIDERATION, THE RECEIPT AND SUFFICIENCY OF
WHICH IS ACKNOWLEDGED, LANDLORD AND TENANT HEREBY AMEND THE LEASE AS FOLLOWS:

        (1)  Section 1.1 of the Lease is amended by striking the words:
                                                    --------

             "Term: Twenty-five (25) years, as the same may be extended as
             provided in Article X hereof." and substituting in their place:
                                                ------------

             "Term: Ten (10) years, as the same may be extended as provided in
             Article X hereof, said ten-year term to commence on the Effective
             Date of the Fourth Amendment to this Lease (as that term is
             defined below).

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        (2)  Section 1.1 of the Lease is amended by striking the words:
                                                    --------

             "Fixed Rent:  $1,517,559 annually, payable monthly in installments
                           of $126,463.25, for the period commencing on the
                           Commencement Date and ending on the last day of the
                           fifth (5th) year thereafter. For the balance of the
                           Term, the Fixed Rent shall be the annual fair market
                           rental value as determined pursuant to Section 4.1 of
                           the Lease."

        and substituting in their place:
            ------------

             "Fixed Rent:  $1,633,135 annually, payable monthly in installments
                           of $136,094.58, for the period commencing on the
                           Effective Date of the Fourth Amendment to this Lease
                           (as that term is defined below) and ending on the
                           last day of the fifth (5th) year thereafter.
                           $1,800,636 annually, payable monthly in installments
                           of $150,053.00, for the period commencing on the
                           fifth anniversary of the Effective Date of the Fourth
                           Amendment to this Lease and ending on the last day of
                           the fifth (5th) year thereafter, the end of the ten
                           year term of this Lease. In the event the Tenant
                           exercises its option to extend the Term of this
                           Lease, all as provided in Article X hereof, the Fixed
                           Rent during any such Term shall be the annual fair
                           market rental value as determined pursuant to and
                           provided for in Article X of the Lease."

        (3)  Section 1.1 of the Lease is amended by adding, at the end of the
section, the following:

             "Effective Date of the Fourth Amendment to this Lease: December 1,
                                                                    ----------
1999."

        (4)  The following additional provision is added to the end of Section
             3.1:

             "As soon as practically possible, and in any event no later than
             September, 2000, Landlord will, at its expense, cause the roof of
             the building on the Premises to be replaced with a new roof at
             least substantially the same in quality as the roof originally
             installed on the building. The replacement roof will be warranted
             by the roof manufacturer or installer, with its standard form of
             warranty for such a roof, for a period of at least ten years
             following installation; Landlord will provide Tenant a document
             from the warrantying party that will recognize Tenant's rights to
             enjoy the benefit of the warranty."

                                       2

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     (5) Section 4.1 of the Lease, except for the first sentence thereof, is
deleted without substitution.

     (6)  Section 5.2.1 of the Lease is amended by deleting the last two
sentences and adding as a final sentence of Section 5.2.1 the following:
              ------

          "Landlord agrees not to withhold unreasonably consent to a sublease of
          all or any portion of the Premises provided that with respect to:

          (i) a sublease of less than 50% of the building area for a term or
              terms less than three years in duration, the proceeds of such
              subletting in excess of the Fixed Rent and Additional Rent are
              shared equally by Landlord and Tenant, and

         (ii) with respect to a subletting of 50% or more of the building area
              or a subletting of all or any portion of the Premises for a term
              of three years or more, that all or any portion of any proceeds
              derived from such subletting in excess of Fixed Rent and
              Additional Rent be paid to Landlord or Landlord may, at Landlord's
              option, elect to terminate this lease with respect to the space
              proposed to be sublet."

     (7) Article X of the Lease is deleted in its entirety and replaced with the
                                   -------
following:

                                   ARTICLE X

                          TENANT'S OPTIONS TO EXTEND


     10.1 Options to Extend.
          -----------------


     (a) Provided that, at the time of each such exercise, (i) this Lease is in
full force and effect, and (ii) no default by Tenant shall have occurred and be
continuing (either at the time of exercise or at the commencement of an Extended
Term), Tenant shall have the right and option to extend the Term of this Lease
for two (2) extended terms (collectively, the "Extended Terms") of five (5)
years each by giving written notice to Landlord not later than twelve (12)
months prior to the expiration date of the Initial Term with respect to the
first Extended Term, and not later than twelve (12) months prior to the
expiration date of the first Extended Term with respect to the second Extended
Term. The effective giving of such notice of extension by Tenant shall
automatically extend the Term of this Lease for the applicable Extended Term,
and no instrument of renewal or extension need be executed. In the event that
Tenant fails timely to give such notice to Landlord, this Lease shall
automatically terminate at the end of the Term then in effect, and Tenant shall
have no further option to extend the Term of this Lease. Each Extended Term
shall commence on the day immediately succeeding the
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expiration date of the Initial Term or the preceding Extended Term, as the case
may be, and shall end on the day immediately preceding the fifth anniversary of
the first day of such Extended Term. The Extended Terms shall be on all the
terms and conditions of this Lease, except: (i) during the Extended Terms,
Tenant shall have no further option to extend the Term, except as provided
herein during the first Extended Term with respect to the second Extended Term,
and (ii) the Fixed Rent for the Extended Terms shall be the Fair Market Rental
Value for the Premises as of the commencement of the Extended Term in question,
determined pursuant to paragraph (b) of this Section 10.1.

     (b)  Promptly after receiving Tenant's notice extending the Term of this
Lease pursuant to paragraph (a) above, Landlord shall provide Tenant with
Landlord's good faith estimate of the Fair Market Rental Value of the Premises
for the applicable Extended Term based upon rents being paid by tenants renewing
leases or entering into new leases for space of comparable size, type, use and
quality in the general vicinity of the Premises, including allowances and
concessions. If Tenant is unwilling to accept Landlord's estimate of Fair Market
Rental Value as set forth in Landlord's notice referred to above, and the
parties are unable to reach agreement hereon within thirty (30) days after the
delivery of such notice by Landlord, then either party may submit the
determination of the Fair Market Rental Value of the Premises to arbitration by
giving notice to the other party naming the initiating party's arbitrator.
Within fifteen (15) days after receiving the notice of initiation of
arbitration, the responding party shall appoint its own arbitrator by notifying
the initiating party of the responding party's arbitrator. If the second
arbitrator shall not have been so appointed within such fifteen (15) day period,
the Fair Market Rental Value of the Premises shall be determined by the
initiating party's arbitrator. If the second arbitrator shall have been so
appointed, the two arbitrators thus appointed shall, within fifteen (15) days
after the responding party's notice of appointment of the second arbitrator,
appoint a third arbitrator. If the two initial arbitrators are unable timely to
agree on the third arbitrator, then either may, on behalf of both, request such
appointment by the Boston office of JAMS/ENDISPUTE, or its successor, or, on its
failure, refusal or inability to act, by a court of competent jurisdiction.
Within fifteen (15) days after the appointment of the third arbitrator, the
three arbitrators shall determine the Fair Market Rental Value of the Premises
and give notice thereof to the parties hereto, and the arbitrators'
determination shall be binding upon the parties. In the event the three
appraisers are unable to agree upon the Fair Market Rental Value of the
Premises, then the average of the two of the three appraisals which are closest
to each other shall be deemed to be the Fair Market Rental Value of the
Premises. In no event shall the annual Fixed Rent to be paid during the first
Extended Term ever be less than the annual Fixed Rent payable during the final
year of the Term prior to extension. In no event shall the annual Fixed Rent to
be paid during the second Extended Term ever be less than the annual Fixed Rent
payable during the final year of the first Extended Term. All arbitrators shall
be appraisers or other qualified real estate professionals who are independent
from the parties and shall be Members of the Appraisal Institute who shall have
had at least ten (10) years commercial real estate experience in the greater
Boston area.

                                       4

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     10.2 No Option to Purchase. Tenant has no right or option whatsoever to
          ---------------------
purchase or otherwise acquire the Premises, any part thereof, or any interest
therein."

     Except as expressly amended in this Amendment, the Lease remains unamended,
and in full force and effect in accordance with its provisions.

     EXECUTED AS A MASSACHUSETTS INSTRUMENT UNDER SEAL AS OF THE DATE FIRST
WRITTEN ABOVE.

                                        LANDLORD

                                        /s/ Francis J. Perry
                                        ------------------------------
                                        Francis J. Perry, Trustee of
                                        Rosemont Trust, as aforesaid,
                                        and not individually

                                        /s/ William J. Walker, Jr.
                                        ------------------------------
                                        William J. Walker, Jr., Trustee of
                                        Rosemont Trust, as aforesaid,
                                        and not individually


                                        TENANT:

                                        LTX CORPORATION

                                        By:  /s/ David G. Facelli
                                        --------------------------------
                                        Name: David G. Facelli
                                        Title: VP/CFO