EXHIBIT 10.b.i

                        The First National Bank of Boston
                               100 Federal Street
                                Boston, MA 02110

                             As of December 6, 1996

USTrust
40 Court Street
Boston, Massachusetts

Attention:   Neal F. Finnegan
             President and Chief Executive Officer

Re: Supplemental Loans, Loan Participations and Assignments
    -------------------------------------------------------

Ladies and Gentlemen:

         Reference  is  hereby  made to that  certain  Purchase  and  Assumption
Agreement,  dated as of June 18, 1996 (as amended or modified  prior to the date
hereof,  the "Purchase  Agreement"),  by and between The First  National Bank of
Boston ("FNBB") and USTrust  ("UST") and joined in for certain limited  purposes
by Bank of Boston Corporation ("BKBC").  Terms defined in the Purchase Agreement
and used  without  definition  herein  shall have the same  respective  meanings
herein as in the Purchase Agreement.

         Pursuant to the terms of the Purchase  Agreement,  FNBB has agreed, and
BKBC has  agreed to cause  BayBank,  National  Association  ("BayBank"),  (i) to
assign to UST on the BayBank  Transfer  Date certain  interests in  Supplemental
Loans (the "Assignments")  and (ii) to grant to UST on the BayBank Transfer Date
certain participating interests in Supplemental Loans (the "Participations"), in
each case as more  fully  set forth in the  Purchase  Agreement  and the  Letter
Agreement,  dated as of the date  hereof,  between  FNBB and UST relating to the
BayBank  Closing (the "BayBank  Closing Side Letter").  In connection  with such
transactions, the parties hereto hereby agree as follows:

          1. FNBB  hereby  agrees to use  reasonable  efforts for a period of 30
days after the BayBank Transfer Date to obtain the consent of Panametrics,  Inc.
in order to change UST's 49%  Participation in the Supplemental Loan relating to
Panametrics,  Inc. to an Assignment of a 49% interest in such  Supplemental Loan
on terms substantially consistent with those contained in the Form of Assignment
and Acceptance attached hereto and incorporated herein as Exhibit A.



                                      - 2 -


          2.  In  the  event  that  FNBB  continues  to be a  lender  under  the
Supplemental Loan relating to Fire Control Instruments, Inc. ("Fire Control") on
March 9, 1997, then UST's participating interest in such Supplemental Loan shall
be reduced from 100% to 49%;  provided,  however,  that, in connection with such
reduction,  FNBB  agrees to use  reasonable  efforts  to cause the  borrower  to
approve a change in UST's 49%  Participation  to an Assignment of a 49% interest
in such  Supplemental  Loan.  FNBB  hereby  acknowledges  and  agrees  that  its
obligations  under the first sentence of the third  paragraph of Section 5(a) of
the  Letter  Agreement,  dated as of  November  8,  1996,  between  FNBB and UST
relating to the split  closings of the sale of the Branches (the "Split  Closing
Side  Letter")  apply  to such  Supplemental  Loan to  Fire  Control;  provided,
however, that notwithstanding the foregoing the parties hereto hereby agree that
the first  sentence of the third  paragraph of Section 5(a) of the Split Closing
Side Letter shall not apply to such  Supplemental  Loan, and FNBB shall not have
any obligation to grant to UST a replacement  Assignment or Participation,  upon
the repayment or prepayment of such Supplemental Loan from proceeds of a loan or
other extension of credit from UST or any Affiliate of UST.

          3. FNBB hereby agrees to use reasonable efforts to determine,  as soon
as  practicable  after  the  BayBank  Transfer  Date and in any  event  within 5
business days after the BayBank Transfer Date,  whether the  Supplemental  Loans
with Ionics,  Incorporated and/or  Panametrics,  Inc. are pledged by FNBB to any
other  person or entity.  In the event that FNBB  determines  that  either  such
Supplemental Loan is so pledged,  then FNBB agrees to use reasonable  efforts to
terminate such pledge,  including,  without limitation,  substituting collateral
for such pledge.  In the event that FNBB  terminates such pledge with respect to
either   Supplemental   Loan,  FNBB  agrees  to  promptly  notify  UST  of  such
termination.  In the event that FNBB is unable to  terminate  such  pledge  with
respect to either  such  Supplemental  Loan  within 15  business  days after the
BayBank Transfer Date (whether by substitution of collateral or otherwise), then
FNBB agrees to repurchase UST's interest in any such Supplemental Loan remaining
pledged and to replace UST's interest in such Supplemental Loan with an interest
in a Replacement Loan (as defined in the applicable  Assignment or Participation
agreement)  in  accordance  with  the  terms  of the  applicable  Assignment  or
Participation  agreement;  provided that, for purposes of determining the timing
of such  repurchase and  replacement,  the date of notice of such repurchase and
replacement  shall be deemed to be the last day of the 15 day period referred to
above.

          4. FNBB agrees to use  reasonable  efforts to, as soon as  practicable
after the  BayBank  Closing  Date,  (a)  provide  UST with  evidence  reasonably
acceptable to counsel for UST indicating  that the $9,800,000  total  commitment
under the 


                                      -3-


Supplemental Loan with HoltraChem,  Inc. has been duly authorized by HoltraChem,
Inc. and (b) either (i) provide UST with evidence  indicating  that the maturity
of such Supplemental  Loan has been duly extended or (ii) obtain an extension of
the maturity of such Supplemental Loan. In the event that FNBB is unable, within
15 business days after the BayBank Transfer Date (or, if requested by UST within
such 15 business day period,  such  additional  period not to exceed 20 business
days), to (A) provide such evidence of due authorization of the total commitment
under such  Supplemental  Loan or (B) (i) provide such evidence of due extension
of the maturity of such  Supplemental  Loan or (ii) if  applicable,  obtain such
extension,  then FNBB agrees to repurchase  UST's interest in such  Supplemental
Loan and to replace UST's interest in such Supplemental Loan with an interest in
a Replacement  Loan (as defined in the  applicable  Assignment or  Participation
agreement)  in  accordance  with  the  terms  of the  applicable  Assignment  or
Participation  agreement;  provided that, for purposes of determining the timing
of such  repurchase and  replacement,  the date of notice of such repurchase and
replacement shall be deemed to be the last day of the 15 day period (as the same
may be extended as set forth above) referred to above.

         The parties  further agree that in the event that FNBB is able,  within
the 15 day  period  set forth  above (as the same may be  extended  as set forth
above),  to provide  such  evidence of due  authorization  and due  extension of
maturity or, if applicable,  extend such maturity, as described in the foregoing
paragraph,  FNBB hereby agrees to use reasonable efforts for a period of 30 days
after the date on which FNBB provides such evidence of due authorization and due
extension of maturity or, if applicable,  extend such maturity, to UST to obtain
the consent of HoltraChem,  Inc. in order to change UST's 49%  Participation  in
the  Supplemental  Loan relating to  HoltraChem,  Inc. to an Assignment of a 49%
interest in such Supplemental Loan on terms substantially  consistent with those
contained  in  the  Form  of  Assignment  and  Acceptance  attached  hereto  and
incorporated herein as Exhibit A.

         5. The parties hereto hereby agree to cooperate with each other in good
faith to enter into, as soon as practicable  and in any event within 10 business
days after the BayBank  Transfer Date,  (a) an Assignment  with respect to a 49%
interest in the Supplemental Loan to Ariad  Pharmaceuticals,  Inc. ("Ariad") and
(b)  mutually   reasonably   satisfactory   arrangements,   including,   without
limitation, documentation reasonably acceptable to counsel to UST and FNBB, with
respect  to  all   outstanding   leases  between  BayBank  and  Ariad  on  terms
substantially similar to the terms of such Assignment or, if an assignment would
be prohibited or require the consent of Ariad under the terms of the  applicable
lease  documents,   on  terms   substantially   similar  to  the  terms  of  the


                                      - 4 -

Participations,  in each case,  applied in the  context of such  leases.  In the
event that such lease  arrangements  are not mutually agreed upon following such
good faith efforts,  then FNBB agrees to replace such  Assignment and such lease
arrangements  with  an  interest  in a  Replacement  Loan  (as  defined  in such
Assignment agreement) in accordance with the terms of such Assignment agreement;
provided  that, for purposes of  determining  the timing of such  repurchase and
replacement,  the date of notice of such  repurchase  and  replacement  shall be
deemed to be the last day of the 10 day period referred to above.

         6. FNBB hereby  agrees that UST shall be entitled,  upon notice to FNBB
no later than 5:00 p.m.,  Boston  time,  on the day after the  BayBank  Transfer
Date, to elect to cause FNBB to repurchase  UST's  interest in the  Supplemental
Loan to RWG Limited Partners, a Massachusetts Limited Partnership ("RWG") if UST
reasonabIy  determines that adequate  collateral is not or may not in the future
be available for such Supplemental  Loan. In the event that UST so elects,  FNBB
shall,  on the  Additional  Residential  Loan  Transfer  Date (as defined in the
BayBank Closing Side Letter) repurchase UST's interest in such Supplemental Loan
and transfer to UST residential loans of a type and quality  reasonably the same
as the  Additional  Residentia1  Loans (as defined in the BayBank  Closing  Side
Letter)  having  an  aggregate   outstanding   principal  amount  on  such  date
approximately  equal to UST's share of the outstanding  principal  amount of the
Supplemental Loan to RWG on such date.

         7. The  parties  hereto  hereby  agree  that,  in the  event  that FNBB
notifies  UST by no later  than 5:00  p.m.,  Boston  time,  on the day after the
BayBank  Transfer Date,  that it has reasonably  determined  that any of (a) the
Assignment  with respect to Carlton  Willard Home,  Inc., (b) the  Participation
with respect to RWG, or (c) the Participation with respect to Newell Properties,
Inc. are  inconsistent  with the terms of (i) Section 5(a) of the Split  Closing
Side Letter or (ii) the loan  documents for the  applicable  Supplemental  Loan,
then the  parties  shall,  as soon as  practicable  and in any event by close of
business on December 10, 1996, cooperate in good faith to modify such Assignment
or Participation in order to make such arrangements consistent with the terms of
Section 5(a) of the Split Closing Side Letter and the applicable loan documents.
In the event that such  modification  is not mutually agreed upon following such
good faith  efforts,  then FNBB  agrees to  repurchase  UST's  interest  in such
Supplemental  Loan and to replace UST's interest in such  Supplemental Loan with
an interest in a Replacement  Loan (as defined in the  applicable  Assignment or
Participation  agreement)  in  accordance  with  the  terms  of  the  applicable
Assignment  or   Participation   agreement;   provided  that,  for  purposes  of
determining the timing of such repurchase and replacement, the date of notice of
such repurchase and

                                       -5-

replacement  shall be deemed to be the date which is 10 business  days after the
BayBank Transfer Date.

         8. FNBB  agrees to  deliver  to  USTrust,  by no later  than 5:00 p.m.,
Boston time,  on the day after the BayBank  Transfer  Date,  the  Attached  Loan
Documents (as defined in the applicable  Participation or Assignment  agreement)
for (a) its  Assignment  with respect to Carlton  Willard  Home,  Inc.,  (b) its
Participation  with  respect to RWG, and (c) its  Participation  with respect to
Newell  Properties,  Inc. The parties hereto hereby agree that FNBB's failure to
deliver such Attached Loan  Documents  prior to such time shall not be deemed to
be a violation of the terms of the applicable Participation or Assignment.

         This letter  agreement  may be executed in any number of  counterparts,
which shall  together  constitute  but one and the same  agreement.  This letter
agreement  shall for all  purposes be governed by, and  construed in  accordance
with,  the  internal  laws of The  Commonwealth  of  Massachusetts.  This letter
agreement  shall take effect as a sealed  instrument as of the date first herein
above  written,  shall be binding  on the  parties  hereto and their  respective
successors and assigns.

                                      - 6 -

         Please  acknowledge  your  agreement  with the foregoing by signing the
enclosed copy of this letter in the place provided below.

                                         Very truly yours,


                                         THE FIRST NATIONAL BANK OF BOSTON

                                         By: /s/ William M. Parent
                                             -----------------------
                                         Title:

ACCEPTED AND AGREED:

USTRUST


By:
   ------------------------
Title:

The undersigned hereby joins this Letter
Agreement   for  the  sole   purpose  of
agreeing  to cause  the  Assets  and the
Assumed Liabilities to be transferred to
UST in  accordance  with the  terms  and
provisions of this Letter  Agreement and
the Purchase Agreement.


BANK OF BOSTON CORPORATION

By: /s/ Peter J. Manning
    -----------------------
Title:


                                      -6-


         Please  acknowledge  your  agreement  with the foregoing by signing the
enclosed copy of this letter in the place provided below.

                                             Very truly yours,

                                             THE FIRST NATIONAL BANK OF BOSTON

                                             By:
                                                ----------------------
                                             Title:


ACCEPTED AND AGREED:

USTRUST

By: /s/ Eric R. Fischer
   ------------------------------
Title: Executive Vice President

The undersigned hereby joins this Letter
Agreement   for  the  sole   purpose  of
agreeing  to cause  the  Assets  and the
Assumed Liabilities to be transferred to
UST in  accordance  with the  terms  and
provisions of this Letter  Agreement and
the Purchase Agreement.

BANK OF BOSTON CORPORATION

By:
   -----------------------------
Title:







                        THE FIRST NATIONAL BANK OF BOSTON
                               100 Federal Street
                           Boston, Massachusetts 02110

                          Dated as of November 8, 1996

USTrust
40 Court Street
Boston, Massachusetts 02108
Attention:   Neal F. Finnegan
             President and Chief Executive Offcer

Re: Amendments to Purchase and Assumption Agreement
    -----------------------------------------------

Gentlemen:

     Reference  is made to the Purchase and  Assumption  Agreement,  dated as of
June  18,  1996  (the  "Purchase   Agreement"),   by  and  between  USTrust,   a
Massachusetts  trust company  ("UST"),  and The First National Bank of Boston, a
national  banking  association  ("FNBB"),  and  joined  in for  certain  limited
purposes by Bank of Boston Corporation, a Massachusetts corporation. Capitalized
terms used herein and not otherwise  defined  herein have the meanings  assigned
thereto in the Purchase  Agreement or in a letter agreement  between the parties
relating to the closing  arrangements  for the  purchase of the Branches of even
date herewith.

     The  parties  hereto  hereby  agree to  amend  the  terms  of the  Purchase
Agreement as follows:

1.  Additional  Definitions.  Section 1.1 of the  Purchase  Agreement  is hereby
amended by inserting,  in the  appropriate  places  designated  by  alphabetical
order, the following new definitions:


          "Merchant  Credit Card  Services"  shall mean the so-called  "Merchant
          Credit Card" services provided to Branch Customers at the Branches.

          "Night  Deposit  Services"  shall  mean  services  relating  to  night
          deposits at the Branches.

          "Silver Storage  Agreements"  shall mean those agreements  relating to
          so-called "silver storage" services at the Branches.




                                      - 2 -

          "Wire  Transfer  Services"  shall  mean  services  provided  to Branch
          Customers relating to wire transfers from the Branches.

2.  Amendments  to Section  1.1 of the  Purchase  Agreement.  Section 1.1 of the
Purchase Agreement is hereby amended by:

         (i)      amending the  definition of "Excluded  Deposits",  by deleting
                  the  word  "and"  at  the  end  of  clause   (f),   inserting,
                  immediately  before the period at the end of clause  (g),  the
                  word "and" and the following new language:

                  (h)  deposits  which are booked or  allocated  to the Branches
                  owned by FNBB and were  established by an "employer"  pursuant
                  to a Keogh Plan. Seller  acknowledges and agrees that Excluded
                  Deposits  shall be excluded from the Deposit  Liabilities  for
                  purposes  of  calculating  the 7% amount  pursuant  to Section
                  3.1(a)  hereof at all times  during  the  period  referred  to
                  therein.

(ii)     amending the definition of "Related  Commercial Products and Services",
         by inserting, immediately  after the phrase "relating to the Commercial
         Loans" in the second line  thereof,  the  following  new phrase,  "and,
         regardless of whether such  services are related to  Commercial  Loans,
         all Merchant  Credit Card  Services,  Night  Deposit  Services and Wire
         Transfer Services"; and

(iii)    amending the  definition  of "Safe Deposit  Agreements",  by inserting,
         immediately after the word "Branches",  the following new phrase,  "and
         Silver Storage Agreements".

3. Amendment to Section 3.3(a) of the Purchase Agreement.  Section 3.3(a) of the
Purchase Agreement is hereby amended by adding the following sentence at the end
thereof:

                  The  parties  agree  that  Arthur  Andersen  LLP  may  act  as
                  Purchaser's agent for purposes of verifying the Purchase Price
                  and Deposit Liabilities  determination in accordance with this
                  section  and shall have the same  access to such work  papers,
                  schedules and other  supporting  data of  FNBB or BayBank with
                  respect  to the  Branches  as  Purchaser  is  entitled  to for
                  purposes of such verification.


                                      - 3 -

4. Amendment to Section 3.3(b) of the Purchase Agreement.  Section 3.3(b) of the
Purchase  Agreement is hereby  amended by deleting the word  "fifteenth"  in the
first line thereof and substituting the word "twentieth" in place thereof.

5. Amendment to Section 3.5(b) of the Purchase Agreement.  Section 3.5(b) of the
Purchase  Agreement  is hereby  amended by deleting the word "and" at the end of
clause (vi) and adding the word "and" at the end of clause  (vii) and adding the
following new clause (viii):

                  (viii)  Periodic  fees  related  to  the  Assets  and  Assumed
                  Liabilities,  including lock box fees, letter of credit annual
                  fees and credit line annual fees.

6.  Amendment  to Section  3.6 of the  Purchase  Agreement.  Section  3.6 of the
Purchase  Agreement is hereby amended by inserting a period after the words "the
provisions  of this  Agreement"  in the third  line  thereof  and  deleting  the
remainder of Section 3.6 in its entirety.

7. Amendment to Section 10.3(e) of the Purchase Agreement. The date "August 31,
1996" in Section  10.3(e) of the Purchase  Agreement  is hereby  deleted and the
date  "November  8,  1996" is hereby  substituted  in place  thereof;  provided,
however,  that  with  respect  to the West  Roxbury  Branch  lease and the Third
Avenue,  Waltham  Branch lease,  the date  referred to in Section  10.3(e) shall
continue  to be  extended by the  parties  beyond  November  8, 1996,  until the
BayBank Closing Date,  provided that there is a reasonable  likelihood that FNBB
will  obtain  the  consents  of  the  landlords  with  respect  to  such  Branch
properties.

8. Amendment to Section  l5.2(a) of the Purchase  Agreement.  Section 15.2(a) of
the  Purchase  Agreement is hereby  amended by deleting the  reference to "12.4"
therein and substituting "16.2" in place thereof.

9.  Amendment  to Section 15.8 of the  Purchase  Agreement.  Section 15.8 of the
Purchase Agreement is hereby amended by inserting, immediately before the period
at the end of the first  sentence  thereof,  a semicolon  and the  following new
phrase:

                  provided,  however,  that this Section 15.8 shall not apply to
                  any negative Deposit to the extent that, on the Transfer Date,
                  Seller transfers to Purchaser (i) positive Deposit Liabilities
                  eligible for offset  against such negative  Deposits or (ii) a
                  credit line which would permit drawings,  in the amount of the
                  negative  Deposit  by the  applicable  Branch  Customers;  and
                  provided further, that the deposit premium calculated pursuant
                  to Section  3.1(a) hereof shall be reduced to reflect any such
                  reduction in such positive  Deposit  Liabilities in accordance
                  with this Section 15.8.



                                      - 4 -

10.  Amendment to Annex A to the Purchase  Agreement.  Paragraph 2 of Annex A to
the  Purchase  Agreement  is hereby  amended by  deleting  the last  sentence of
Paragraph 2 in its entirety and  inserting  the  following new sentence in place
thereof:

                  "Purchaser and Seller agree that, at Seller's expense (for the
                  increase and incremental  employment taxes attributable to the
                  increase),  the retention  bonus shall be increased by 1.7% of
                  the  base  amount  specified  above,   resulting  in  a  total
                  retention bonus of 10% of such base amount."

11. Certain  Transferred  Deposits.  UST  acknowledges and agrees that, with its
knowledge and consent at the request of the applicable  Branch  Customer,  or as
otherwise  agreed  to by the  parties  based on the  fact  that a  customer  has
relocated  from  the  market  area  of  the  Branches  or has a  "home  banking"
relationship  with FNBB or BayBank,  certain  Deposits have been  transferred by
FNBB and BayBank  from the  Branches to other bank  branches of FNBB and BayBank
not being  acquired  by UST (all such  deposit  accounts  delivered  hereinafter
referred to as "Transferred  Deposits").  UST  acknowledges and agrees that such
transfers  shall be deemed not to have been made in  violation  of the  Purchase
Agreement.  FNBB acknowledges and agrees that such Transferred Deposits shall be
excluded from the Deposit  Liabilities for purposes of calculating the 7% amount
pursuant to Section  3.1(a) of the  Purchase  Agreement  at all times during the
period  referred to therein.  As of the date hereof,  FNBB  represents that such
Transferred Deposits have not in the aggregate exceeded $8,000,000. FNBB and UST
also  acknowledge  and agree that,  for  purposes of  calculating  the 7% amount
pursuant to Section 3.1 (a) of the Purchase  Agreement,  the Deposit Liabilities
shall at all times  during  the  period  referred  to  therein  include  Deposit
Liabilities  in accounts  that have been or are  transferred  by FNBB or BayBank
into the Branches prior to the applicable Transfer Date.

12. Certain Related Comercial Products and Services. The parties acknowledge and
agree that certain accounts  relating to (i) repurchase  agreements with certain
customers of FNBB, having an aggregate balance of approximately $3 million as of
the date hereof and (ii) sweep arrangements with Commercial Loan customers known
as "BayBank Precision Sweep Accounts"  maintained or allocated by BayBank at the
BayBank  Branches,  and having an aggregate balance of approximately $60 million
as of the date hereof,  constitute Related Commercial  Products and Services for
purposes of the Purchase Agreement and will be transferred to UST, together with
an amount equal to the aggregate  amount due to such  customers  with respect to
such  agreements  and  accounts,  on the  FNBB  or  BayBank  Transfer  Date,  as
applicable;  provided,  however,  that such  repurchase  agreements  and BayBank
Precision Sweep Accounts shall not be deemed Deposit Liabilities for purposes of
computing the 7% amount under Section  3.1(a) of the Purchase  Agreement and for
purposes of Sections 3.2 and 3.3 of the Purchase Agreement.

                                      - 5 -

13.  Agreements as to Certain  Branch  Leases.  Notwithstanding  anything to the
contrary in the Purchase  Agreement,  the parties  hereby agree as follows:

         (i) West Roxburv  Branch Lease.  The parties hereby agree that (a) FNBB
         shall  assign and UST shall  accept the  Branch  Lease for the  BayBank
         Branch  located in West  Roxbury  and (b) the  Purchase  Price shall be
         reduced  by an  amount  equal to  [$69,436.19],  the  present  value of
         one-half of the  additional  rental  payments  required by the landlord
         after the BayBank  Closing Date for the  remainder of the current lease
         term.

         (ii) Third Avenue,  Waltham Branch Lease. The parties hereby agree that
         the  Branch  Lease for the  BayBank  Branch  located  at Third  Avenue,
         Waltham  includes the lease of the office  space  located on the second
         and third floors of the Third Avenue,  Waltham  Branch (the "Second and
         Third Floor Lease") and that (a) FNBB shall assign and UST shall accept
         the Third Avenue,  Waltham Branch Lease, including the Second and Third
         Floor Lease, (b) the Purchase Price shall be reduced by an amount equal
         to  $610,079.22,  one-half  the  present  value of the rental  payments
         required to be paid from the BayBank  Closing Date for the remainder of
         the  current  lease term for the Second and Third  Floor  Lease and (c)
         after the Bay Bank Closing Date,  UST shall,  upon receipt  thereof (in
         the case of a sublease) or at the end of each month (in the case of use
         by UST),  pay FNBB an amount  which is equal to  one-half of any rental
         payments  received  by UST if the  Second  and  Third  Floor  Lease  is
         subleased to a third party or one-half of the fair market value, taking
         into account the nature of any such utilization (e.g. retail, office or
         storage)  of the rental of that  portion of the Second and Third  Floor
         Lease  space,  if any,  utilized by UST or any of its  Affiliates.  UST
         agrees to obtain the consent of FNBB prior to making any alterations or
         improvements  with  respect to the Second and Third  Floor  Lease space
         which consent,  unless the alterations or improvements  are made at any
         time when the  remaining  term on the Second and Third  Floor  Lease is
         less than two years and would result in an expense to FNBB  pursuant to
         the  immediately  succeeding  sentence  of an  amount  in excess of the
         amount of the  aggregate  yearly  rent under such  lease,  shall not be
         unreasonably  withheld or delayed.  In the event that, with the consent
         of FNBB, UST makes  alterations or improvements to the Second and Third
         Floor Lease space in  connection  with any  sublease of such space to a
         third party,  FNBB agrees to reimburse UST for one-half of the expenses
         incurred  by UST in  connection  therewith  (net of any portion of such
         expenses  paid by any other Person such as the owner of the sublease of
         such space).

14.  Agreement as to Certain "ATM  Surrounds".  Notwithstanding  anything to the
contrary in the Purchase Agreement,  the parties hereby agree that FNBB shall be
entitled, at its own expense, to remove all "surrounds" from the ATMs located at
the  Branches;  provided,  however,  that FNBB hereby  agrees not to remove such
surrounds  until the  expiration  of the ninety day  period  following  the FNBB
Transfer Date, as applicable, or a

                                      - 6 -

reasonable  time thereafter to permit UST to replace such "ATM Surrounds" at the
Branches;  and  provided  further,  that (i) upon the  removal  of any such "ATM
Surround" by FNBB from any Branch subject to a Branch Lease,  UST shall promptly
replace  such "ATM  Surround,"  and (ii) to the extent any  landlord  consent is
required for such removal of any "ATM  Surrounds",  FNBB shall,  at its expense,
obtain  such  consents  and UST shall use  reasonable  efforts to assist FNBB in
obtaining such consents.

15.  Agreement as to Branch  Closing  Times.  The parties  hereby agree that the
closing of banking  business to the public at the FNBB  Branches and the ATMs at
the FNBB Branches on the FNBB Closing Date shall be 4 p.m. and  approximately  3
p.m.,  respectively.  The  parties  hereby  agree  that the  closing  of banking
business to the public at the BayBank  Branches and ATMs at the BayBank Branches
shall be 4 p.m. and 4 p.m., respectively.

16.  Waiver of certain  Purchaser  Covenant in Section  15.2(a) of the  Purchase
Agreement.  FNBB  acknowledges and agrees that UST shall distribute within three
(3) business days after the Closing to customers of FNBB whose deposit  accounts
are being  transferred  to UST, an initial  supply of new basic checks,  deposit
tickets or other similar instruments and ATM cards. FNBB acknowledges and agrees
that the  failure by UST to deliver  such items at least seven (7) days prior to
the Transfer Date as prescribed by the Purchase Agreement shall not be deemed to
be a violation of the Purchase Agreement.

17. Agreement as to Merchant Credit Card Machines. FNBB hereby agrees to use all
reasonable efforts to cause BayBank to transfer to UST all of BayBank's interest
in certain  Merchant  Credit Card machines  owned or leased by BayBank,  in each
case at a price to be mutually agreed upon by the parties hereto.

18. Cash Management Services.   Simultaneously  herewith, the parties hereto are
entering into a Cash Management Services  Agreement,  pursuant to which FNBB has
agreed to provide certain cash management services to UST after the FNBB Closing
Date.

19. Advance Accounts.  Notwithstanding anything in the Purchase Agreement to the
contrary, the parties hereto hereby agree that FNBB and BayBank may transfer out
of the  applicable  Branches  to other  branches  of FNBB or  BayBank  not being
acquired by UST,  any  overdraft  lines of credit which are more than sixty (60)
days delinquent,  together with the deposit account to which such overdraft line
of credit relates.  The parties  acknowledge and agree that such transfers shall
be  deemed  not to have  been made in  violation  of the  terms of the  Purchase
Agreement.  FNBB  acknowledges  and agrees  that  deposit  accounts  transferred
pursuant to this Paragraph 19 shall be excluded from the Deposit Liabilities for
purposes  of  calculating  the 7%  amount  pursuant  to  Section  3. l(a) of the
Purchase Agreement at all times during the period referred to therein.


                                      -7-

20. Certain Leased Equipment.  Simultaneously herewith,  BancBoston Leasing Inc.
is selling and UST Leasing  Corporation is purchasing certain equipment owned by
BancBoston Leasing Inc. and leased to certain Commercial Loan customers.

21. Nonsolicitation Guidelines.  Subsection (c) of the third sentence of Section
10.4 of the Purchase  Agreement is hereby amended by deleting such subsection in
its entirety and inserting the following new subsection (c) in place thereof:

         (c) respond to, or offer FNBB or BayBank  products and for services to,
         Branch Customers,  including, without limitation,  Branch Customers who
         retain  deposit  accounts at FNBB or BayBank  after the FNBB or BayBank
         Transfer Date ("Split  Customers") during or in response to unsolicited
         in-person  or  telephonic   inquiries  by  Branch  Customers  or  Split
         Customers  with  respect  to  banking  or  other  financial   services,
         including without limitation, Commercial Loans; and

22. Agreement as to Certain Transitional  Matters.  Notwithstanding  anything in
the  Purchase  Agreement to the  contrary,  the parties  hereto  hereby agree as
follows with respect to certain Deposit histories of the Branch Customers:

         (i) In the case of any bona fide dispute  between a Branch Customer and
         UST  concerning  the servicing of a Deposit  account by FNBB or BayBank
         relative to the period  prior to the  transfer of such  account to UST,
         FNBB will  provide or cause  BayBank  to  provide to UST,  at FNBB's or
         BayBank's expense,  as applicable,  to the extent reasonably  requested
         and  available,  information  and copies of documents  relating to such
         Deposit  account in a timely  manner which would  comply with  standard
         banking practices and customs.  UST agrees that it shall reimburse FNBB
         or  BayBank,  as  applicable,  for all direct  costs  incurred  by such
         persons in connection with providing such information  and/or copies to
         the  extent  that UST  would  customarily  charge  a fee to the  Branch
         Customer in connection  with UST's  providing such  information  and/or
         copies;

         (ii)  In the  event  that  UST  receives  a  subpoena  or is  otherwise
         requested  pursuant  to legal  process or  judicial  or  administrative
         proceedings to provide  information  and/or  documents  relating to the
         servicing of a Deposit account by FNBB or BayBank prior to the transfer
         of such  account to UST, UST shall  request  that the party  requesting
         such  information  subpoena or request by legal  process or judicial or
         administrative proceedings that FNBB or BayBank, as applicable, provide
         such  information  or  documentation.  In the event that the requesting
         party  refuses or is unable to subpoena or otherwise  make such request
         to FNBB or BayBank  directly,  then,  at the request of UST,  FNBB will
         provide or cause BayBank to provide, at




                                      - 8 -



         FNBB's  or  BayBank's  own  expense,  to  the  extent  available,  such
         information or documentation in a timely manner which would comply with
         standard banking practices and customs; and

         (iii) In the case of any inquiry by a Branch  Customer  relating to the
         servicing of a Deposit account by FNBB or BayBank prior to the transfer
         of such account to UST (but not relating to a bona fide dispute between
         such Branch Customer and UST), or otherwise upon request by UST (except
         to the extent  provided in paragraphs (i) and (ii) above),  FNBB agrees
         to provide UST, to the extent reasonably requested and available,  with
         information and copies of pertinent  documents relating to such account
         in a timely manner which would comply with standard  banking  practices
         and  customs,  and UST  agrees  to  reimburse  FNBB for  FNBB's  direct
         expenses  incurred in providing any such  information  and/or copies of
         documents.

23.  Miscellaneous.  Except  as  expressly  set  forth  herein,  all  terms  and
conditions of the Purchase Agreement are hereby ratified and confirmed and shall
remain in full force and effect,  and each party hereto expressly affirms all of
its  obligations  under the Purchase  Agreement.  This Letter  Agreement  may be
executed in one or more  counterparts,  all of which shall be considered one and
the same agreement, and this Letter Agreement shall become effective when one or
more  counterparts  have been signed by each of the parties hereto.  This Letter
Agreement  shall be governed by and construed in accordance with the laws of The
Commonwealth of Massachusetts  (without  reference to conflicts or choice of law
provisions).





                                      - 9 -


     Please  acknowledge  your  agreement  with the  foregoing  by  signing  the
enclosed copy of this letter in the place provided below.

                                             Very truly yours,

                                             THE FIRST NATIONAL BANK OF BOSTON

                                             By: /s/ William M. Parent
                                                 -------------------------
                                             Title:

ACCEPTED AND AGREED:

USTRUST

By: /s/ James K. Hunt
    ------------------
Title:

The undersigned hereby joins this Letter
Agreement   for  the  sole   purpose  of
agreeing  to cause  the  Assets  and the
Assumed Liabilities to be transferred to
UST in  accordance  with the  terms  and
provisions of this Letter  Agreement and
the Purchase  Agreement.

BANK OF BOSTON CORPORATION


By: /s/ Peter J. Manning
    ---------------------
Title: