CONTRACT B-1
                                  CONFIRMATION


Date:                      May 12, 1997

To:                        Interneuron Pharmaceuticals, Inc. ("Party B")

Attention:                 Thomas F. Farb

From:                      Swiss Bank Corporation, London Branch ("Party A")

Re:                        Equity Option Confirmation
                           Reference Number___________________
--------------------------------------------------------------------------------
The purpose of this  communication is to confirm the terms and conditions of the
transaction  (the  "Transaction")  entered  into  between  us on the Trade  Date
specified below. This  Confirmation  constitutes a "Confirmation" as referred to
in the 1992 ISDA Master Agreement specified below.

The  definitions  and  provisions  contained  in the 1991 ISDA  Definitions  (as
published by the International Swaps and Derivatives Association, Inc. (formerly
known  as the  International  Swap  Dealers  Association,  Inc.)  ("ISDA"))  are
incorporated into this Confirmation.  In the event of any inconsistency  between
those definitions and provisions and this  Confirmation,  this Confirmation will
govern.

This Confirmation supplements, forms part of, and is subject to, the ISDA Master
Agreement dated as of May 5, 1997, as amended and supplemented from time to time
(the  "Agreement"),  between  you and Swiss  Bank  Corporation.  All  provisions
contained in the Agreement govern this Confirmation except as expressly modified
below.

The terms of the Transaction to which this Confirmation relates are as follows:

Trade Date                 :        May 9, 1997

Buyer                      :        Party A

Seller                     :        Party B

Option Style               :        European Option

Option Type                :        Call

Shares            :        Common Stock of Interneuron Pharmaceuticals, Inc.  
                           (Symbol: IPIC)

Number of Options          :        1,000,000

Contract Multiplier        :        1.00

Strike Price               :        USD 40.30

Total Premium     :        Call options  written by  Party A on shares of  Party
                           B's Common Stock.

Premium
Payment Date      :        May 9, 1997

Expiration  Date  :        May 21,  1999,  or, if that  date is not an  Exchange
                           Business  Day, the  following day that is an Exchange
                           Business  Day;  provided,  however,  that if  Party B
                           elects to cash settle the  Transaction as hereinafter
                           provided,  the  Expiration  Date shall be extended to
                           the 35th Exchange Business Day after such date.

Currency









Business  Day    :         Any  day on  which  commercial  banks  are  open  for
                           business  (including dealings in foreign exchange and
                           foreign  currency  deposits) in the cities from which
                           and in which a payment is to be made.

Exchange
Business Day     :         A day that is (or but for the  occurrence of a Market
                           Disruption  Event,  would have been) a trading day on
                           the  Exchange  (other than a day on which  trading on
                           any such  exchange is scheduled to close prior to its
                           regular weekday closing time,  first announced on the
                           day of such closing).

Normal
Trading Day     :          An   Exchange   Business   Day  on  which  no  Market
                           Disruption Event has occurred or is continuing.

Market
Disruption Event  :        The occurrence or existence on any Exchange  Business
                           Day during the one-half  hour period that ends at the
                           close of business of any  suspension of or limitation
                           imposed on trading (by reason of  movements  in price
                           exceeding levels  permitted by the relevant  exchange
                           or  otherwise),  provided  that  any  such  event  is
                           material  in  the  reasonable  determination  of  the
                           Calculation  Agent, and agreed to by Party B, on: (i)
                           the Exchange in the Shares; or (ii) the Pacific Stock
                           Exchange in options contracts on the Shares.

Exchange          :        Nasdaq National Market

Clearance System
Business Day      :        Any day on which the Clearance System is open for the
                           acceptance and execution of settlement instructions.

Clearance System  :        Depository  Trust  Company,  or any  successor  to or
                           transferee of such clearance system.

Calculation Agent :        Party A, whose  calculations  shall be binding absent
                           manifest error.

Procedure for Exercise
----------------------

Exercise Date     :        The Expiration Date.

Expiration Time   :        5:00 p.m. local time in New York City

                           Automatic  Exercise : The Transaction  will be deemed
                           to be  automatically  exercised if it is In-the-Money
                           on the  Expiration  Date,  unless  (i) the  Buyer has
                           notified  the Seller  (by  telephone  or in  writing)
                           prior to 5:00 p.m. local time in New York City on the
                           Expiration Date that it does not wish to exercise the
                           Transaction (or wishes to delay the Expiration  Date,
                           as set forth below); or (ii) the Closing Value cannot
                           be  determined  on  the   Expiration   Date.  If  the
                           Transaction  is to  be  cash  settled  or  net  share
                           settled,   "In-the-Money"   means   that   the   Cash
                           Settlement  Amount  is  greater  than  zero.  If  the
                           Transaction    is   to   be    physically    settled,
                           "In-the-Money"   means  that  the  Closing  Value  is
                           greater than the Strike Price.  "Closing Value" means
                           the closing  price of the Shares,  as reported on the
                           Exchange, on the Expiration Date.

Seller's telephone
or facsimile number

                                       2






for purposes of
giving notice         :        Telephone:       (617) 861-8444 ext. 607
                                    Fax:             (617) 674-2448
                                    Attention:       Thomas F. Farb



Settlement Terms
----------------

Settlement            :    The Transaction will be physically settled; provided,
                           however, that Party B may elect to cash settle or net
                           share  settle  the  Transaction  by giving  notice to
                           Party A no  later  than  30  Exchange  Business  Days
                           before  the  Expiration  Date.   Notwithstanding  the
                           foregoing,  if on or  prior  to  the  fifth  Exchange
                           Business  Day  prior  to  the  Expiration  Date,  the
                           registration  statement referred to in Section 4.1 of
                           the  Agreement  Regarding   Registration  Rights  and
                           Related  Obligations (the  "Registration  Agreement")
                           attached   hereto  as   Exhibit  B  has  not   become
                           effective,  the Transaction  will be cash settled and
                           the parties shall have no further  obligations  under
                           the Registration Agreement.

Physical Settlement   :    If the Transaction is to be physically settled:

                           (a) at such time as Party B requests  but in no event
                           later  than 20  Exchange  Business  Days  before  the
                           Expiration  Date, Party A and Party B will enter into
                           the Registration Agreement attached hereto as Exhibit
                           I; and

                           (b) on the Settlement  Date, the Seller shall deliver
                           to the  Buyer  the  number  of  Shares  equal  to the
                           Contract  Multiplier  multiplied  by  the  number  of
                           Options exercised against payment by the Buyer to the
                           Seller of an amount  equal to the  product of (A) the
                           Strike   Price   multiplied   by  (B)  the   Contract
                           Multiplier  multiplied  by (C) the  number of Options
                           exercised.  Such  payment and such  delivery  will be
                           made  through the  Clearance  System at the  accounts
                           specified below, on a delivery versus payment basis.

Cash Settlement   :        If the Transaction is to be cash settled:

           (a)      within  one  Currency  Business  Day  after  written  notice
                           by  Party  A,  Party  B  will  pay  to  Party  A  the
                           Prepayment Amount.  The "Prepayment  Amount" shall be
                           an amount  calculated by the Calculation  Agent equal
                           to (i) the Contract Multiplier multiplied by (ii) the
                           number of Options to be exercised multiplied by (iii)
                           the result of  subtracting  the Strike Price from the
                           closing  price  of the  Shares,  as  reported  on the
                           Exchange, on the date of such notice by Party A.

           (b)      on the Settlement Date, the following payment shall be made:

                                 (i) if a  Prepayment  Amount  has been  paid by
                                 Party B, then (A) if the Final  Payment  Amount
                                 is a  positive  number,  Party B  shall  pay to
                                 Party A an amount  equal to the  Final  Payment
                                 Amount and (B) if the Final Payment Amount is a
                                 negative  number,  Party A shall pay to Party B
                                 an amount  equal to the  absolute  value of the
                                 Final  Payment   Amount.   The  "Final  Payment
                                 Amount"  shall be an amount,  which may be less
                                 than zero,  calculated by the Calculation Agent
                                 equal to (A) the Cash  Settlement  Amount minus
                                 (B)  the   Prepayment   Amount  minus  


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                                 (C) the Interest  Amount.  The "Cash Settlement
                                 Amount"  shall be the  greater  of (A) zero and
                                 (B) an  amount  calculated  by the  Calculation
                                 Agent  equal  to (1)  the  Contract  Multiplier
                                 multiplied   by  (2)  the   number  of  Options
                                 exercised  multiplied  by  (3)  the  result  of
                                 subtracting the Strike Price from the Reference
                                 Price. The "Interest Amount" shall be an amount
                                 calculated  by the  Calculation  Agent equal to
                                 the  aggregate  sum of the  amounts of interest
                                 calculated for each day in the period from (and
                                 including)  the date the  Prepayment  Amount is
                                 received  by  Party  A to (but  excluding)  the
                                 Settlement Date, determined as follows: (A) the
                                 Prepayment   Amount   multiplied   by  (B)  the
                                 overnight  Federal  Funds  Rate for such day as
                                 reported   in   Federal   Reserve   Publication
                                 H.15-519 divided by (C) 360.

                                 (ii) if a  Prepayment  Amount has not been paid
                                 by  Party B,  Party B shall  pay to Party A the
                                 Cash Settlement Amount, if any.

Reference Price   :        (a)  If  the  Valuation  Period  contains  35  Normal
                           Trading  Days,  the  Reference  Price  shall  be  the
                           arithmetic  average  of the Share  Prices on those 35
                           Normal Trading Days.

         (b) If the  Valuation  Period does not contain 35 Normal  Trading Days,
                           Party A and  Party B,  shall  jointly  determine  the
                           Share  Price  for  the  Valuation  Date  and as  many
                           Exchange  Business  Days  immediately  preceding  the
                           Valuation Date as shall be necessary, when such Share
                           Prices are taken  together  with the Share  Prices on
                           all  Normal   Trading  Days   occurring   within  the
                           Valuation Period, to provide 35 Share Prices,  and in
                           such case the Reference Price shall be the arithmetic
                           average of those 35 Share Prices.

Share Price      :         The volume  weighted  average  price of the Shares as
                           reported on  Bloomberg  for the period from 9:30 a.m.
                           to 4:30 p.m. local time in New York City on the given
                           day.

Valuation Period :         The  period  from and  including  the  34th  Exchange
                           Business Day  immediately  preceding  the  Expiration
                           Date  (the  "Initial  Date")  to  and  including  the
                           Expiration  Date,   provided  that  if  any  Exchange
                           Business   Day  in  the   Valuation   Period   as  so
                           determined,  shall not be a Normal  Trading  Day, the
                           Valuation  Period  shall  be  extended  so  that  the
                           Valuation Period includes 35 Normal Trading Days, but
                           in no  event  shall  the  last  day of the  Valuation
                           Period be later than the 35th  Exchange  Business Day
                           after the Expiration  Date, and in no event shall the
                           Valuation  Period  include any day before the Initial
                           Date.

Valuation Date    :   The last day of the Valuation Period.

Net Share
Settlement        :        If a Transaction is to be net share settled,

                           (a) no later than 20  Exchange  Business  Days before
                           the Expiration  Date,  Party A and Party B will enter
                           into the  Registration  Agreement  attached hereto as
                           Exhibit I; and

                (b) on the Settlement Date, Party B shall deliver to Party A the
                           number  of whole  Shares  (the  "Settlement  Shares")
                           equal to (i) the Cash  Settlement  Amount  divided by
                           (ii)  the  Closing  Value,  plus  cash in lieu of any
                           fractional  Shares.  If within 


                                       4






                           ten Exchange Business Days after the Settlement Date,
                           Party A resells all or any portion of the  Settlement
                           Shares and the net  proceeds  received by the Party A
                           upon resale of such Shares exceed the Cash Settlement
                           Amount (or if less than all of the Settlement  Shares
                           are resold,  the  applicable  pro rata portion of the
                           Cash  Settlement  Amount),  Party  A  shall  promptly
                           refund  in cash  such  difference  to Party B. In the
                           event that such net  proceeds  are less than the Cash
                           Settlement  Amount  (or  if  less  than  all  of  the
                           Settlement Shares are resold, the applicable pro rata
                           portion of the Cash Settlement Amount), Party B shall
                           pay in cash or additional Shares such difference (the
                           "Make-whole   Amount")  to  Party  A  promptly  after
                           receipt of notice thereof.  In the event that Party B
                           elects to pay the  Make-whole  Amount  in  additional
                           Shares,  Party B shall  deliver to Party A the number
                           of whole Shares (the  "Make-whole  Shares")  equal to
                           (i) the Make-whole Amount divided by (ii) the closing
                           price of the Shares as  reported  on the  Exchange on
                           the  Exchange  Business Day prior to delivery of such
                           Shares.  If within ten Exchange  Business  Days after
                           the  delivery  of the  Make-whole  Shares to Party A,
                           Party A resells all or any portion of such Shares and
                           the net  proceeds  received  by Party A exceed or are
                           less than the Make-whole  Amount (or if less than all
                           of the Make-whole  Shares are resold,  the applicable
                           pro  rata  portion  of the  Make-whole  Amount),  the
                           provisions set forth above with respect to payment of
                           the   Settlement   Payment   in   Shares,   including
                           Make-whole requirements, shall apply.

Settlement Date     :      If  the  Transaction  is  to  be  cash  settled,  the
                           Settlement Date shall be three Currency Business Days
                           after the Valuation Date. If the Transaction is to be
                           physically   settled  or  net  share   settled,   the
                           Settlement  Date  shall  be  three  Clearance  System
                           Business Days after the Exercise Date.

Change in Expiration Date
-------------------------

Delay or Acceleration
of Expiration Date  :      At any time  prior to the day  that  otherwise  would
                           have been the Initial Date,  Party B may notify Party
                           A that it  would  like to  delay  or  accelerate  the
                           Expiration  Date,  as  of a  date  (the  "Termination
                           Date") not less than ten Exchange Business Days after
                           the date such  notice,  and,  in the case of a delay,
                           not more  than 180 days  after the  Expiration  Date.
                           Promptly after receipt of such notice,  Party A shall
                           notify  Party  B of the  additional  amount,  if any,
                           required to be paid (the "Settlement  Amount") (i) in
                           the case of a delay of the  Expiration  Date,  by the
                           Buyer of the  Transaction to compensate the Seller of
                           the  Transaction  for such delay, or (ii) in the case
                           of an  acceleration  of the  Expiration  Date, by the
                           Seller of the  Transaction to compensate the Buyer of
                           the Transaction for such acceleration. The Settlement
                           Amount  shall  be  reasonably  determined  by Party A
                           based upon  Party A's then  current  methodology  for
                           pricing options.  Within five Exchange  Business Days
                           after receipt of notice of the Settlement Amount from
                           Party A,  Party B shall  notify  Party A  whether  it
                           agrees  to the  Settlement  Amount  (the date of such
                           notice,  the "Agreement  Date"). If Party B agrees to
                           the Settlement  Amount (i) the Termination Date shall
                           be deemed to be the Expiration  Date for all purposes
                           of this Confirmation; and (ii) the Settlement Amount,
                           which   shall  be  in  addition  to  any  payment  or
                           delivery, if any, otherwise required to be made under
                           the  terms of the  Transaction,  shall be paid by the
                           appropriate  party on the third Currency Business Day
                           after the  Agreement  Date. If Party B does not agree
                           to the Settlement  Amount,  the Expiration Date shall
                           not be delayed or accelerated, as the case may 


                                       5





                           be.

Adjustment Events
-----------------

Adjustments        :       During the life of the Transaction, if any adjustment
                           is made by The Options  Clearing  Corporation  or its
                           successors   ("OCC")  in  the  terms  of  outstanding
                           OCC-issued  options  ("OCC  Options")  on the  Shares
                           which  are  the  subject  of  the  Transactions,   an
                           equivalent  adjustment  shall be made in the terms of
                           the Transaction.  Except as provided in the following
                           paragraph   and   in   the   "Additional   Adjustment
                           Provisions" below, no adjustment shall be made in the
                           terms of the  Transaction for any event that does not
                           result in an adjustment  to the terms of  outstanding
                           OCC  Options  on the  Shares.  Without  limiting  the
                           generality of the foregoing,  NO ADJUSTMENT  SHALL BE
                           MADE IN THE TERMS OF THE  TRANSACTIONS  FOR  ORDINARY
                           CASH  DIVIDENDS  ON THE SHARES  EXCEPT AS PROVIDED IN
                           THE "ADDITIONAL ADJUSTMENT PROVISIONS" BELOW.

                           If at any time  during  the  life of the  Transaction
                           there  shall be no  outstanding  OCC  Options  on the
                           Shares,  and  an  event  shall  occur  for  which  an
                           adjustment might otherwise be made under the By-Laws,
                           Rules,  and stated  policies of the OCC applicable to
                           the  adjustment  of OCC Options (the "OCC  Adjustment
                           Rules"),  the parties  shall use their best  efforts,
                           applying  the   principles   set  forth  in  the  OCC
                           Adjustment  Rules,  to jointly  determine  whether to
                           adjust the terms of the Transaction and the nature of
                           any such adjustment.

Additional
Adjustments        :       Notwithstanding the foregoing, if upon the occurrence
                           of  the  following  events  during  the  life  of the
                           Transaction  no  adjustment is required to be made to
                           the terms of the  Transaction in accordance  with the
                           foregoing  provisions  or if an  adjustment  has been
                           made  but  such  adjustment  is,  in  the  reasonable
                           determination   of   the   Calculation    Agent,   in
                           consultation  with Party B,  insufficient to preserve
                           the  economic  benefit  of the  Transaction  for  the
                           parties, the following  additional  adjustments shall
                           be made to the terms of the Transaction:

                           (a) If (i) an ordinary  cash  dividend is declared or
                           paid on the Shares or (ii) a special cash dividend is
                           declared or paid on the Shares and in either case the
                           Ex-Dividend Date with respect to such dividend occurs
                           during the period from, and including,  the Effective
                           Date to, but excluding,  the Expiration Date (each, a
                           "Dividend Event"),  the Strike Price and the Contract
                           Multiplier  shall each be adjusted for each  Dividend
                           Event in accordance with the following formulas:

                                               SP0 (CP0 - DA)
                                   SP1    =  ------------------         
                                                    CP0

                                                 SP0
                                   CM1    =  ------------  
                                                 SP1

                                   Where:

                           (i)     SP1 = Strike Price after the Dividend Event

                           (ii)    CM1 = Contract Multiplier after the Dividend 
                                         Event

 
                                        6






                           (iii)   SP0 = Strike Price on the  Exchange  Business
                                         Day    immediately     preceding    the
                                         Ex-Dividend  Date with  respect  to the
                                         Dividend Event

                           (iv)    CP0 = The  closing  price of the  Shares,  as
                                         reported  on  the   Exchange,   on  the
                                         Exchange   Business   Day   immediately
                                         preceding  the  Ex-Dividend  Date  with
                                         respect to the Dividend Event

                           (vi)    DA =  The   amount  of  the   ordinary   cash
                                         dividend or the special cash  dividend,
                                         as the case may be

                           (b) If there occurs a tender offer,  by the issuer of
                           the Shares (the  "Issuer") or a third party,  for the
                           Shares,  and the date of the expiration of such offer
                           (the  "Termination  Date")  occurs  during the period
                           from,  and  including,  the  Effective  Date to,  but
                           excluding,   the  Expiration   Date  (an  "Adjustment
                           Event"), the Strike Price and the Contract Multiplier
                           shall each be adjusted for each  Adjustment  Event in
                           accordance with the following formulas:

                                                    (CP0  x N0)  - C
                                                  --------------------
                                  SP1  =  SP0  x  [ CP0 x (N0  - N1) ]

                                           SP0
                                  CM1  =  ----- 
                                           SP1

                                  Where:

                           (i)    SP1 = Strike Price after the Adjustment Event

                           (ii)   CM1 = Contract Multiplier after the Adjustment
                                        Event

                           (iii)  SP0 = Strike Price on the  Completion Date (as
                                        hereinafter defined)

                           (iv)   CP0 = The  closing  price  of the  Shares,  as
                                        reported   on  the   Exchange,   on  the
                                        Completion Date

                           (v)     N0 = Total  number of shares of common  stock
                                        of  the   Issuer   outstanding   on  the
                                        Completion Date

                           (vi)    C  = Price paid by the  offeror  per share of
                                        common stock of the Issuer multiplied by
                                        the total number of shares  purchased by
                                        the offeror  pursuant to the  Adjustment
                                        Event

                           (vii)   N1 = Total  number of shares of common  stock
                                        of the Issuer  purchased  by the offeror
                                        pursuant to the Adjustment Event

                           (viii)  "Completion Date" means the Termination Date;
                                   provided,   however,   that  if  the   period
                                   following the  Termination  Date within which
                                   delivery of tendered shares may be guaranteed
                                   is less than the customary  settlement period
                                   for a sale of the Shares executed through the
                                   Clearance  System,  the Completion Date shall
                                   be the date which  precedes  the  Termination
                                   Date by the number of Exchange  Business Days
                                   equal to such difference


                                       7




Miscellaneous
-------------

Transfer           :       Neither party may transfer the Transaction,  in whole
                           or in part,  without the prior written consent of the
                           non-transferring party.

Account Details
---------------

Payments and deliveries to Party A:     Previously provided

Payments and deliveries to Party B:     Previously provided


                                       8






Please  confirm  that  the  foregoing  correctly  sets  forth  the  terms of our
agreement by executing the copy of this  Confirmation  enclosed for that purpose
and  returning  it to us or by  sending  to us a letter  or telex  substantially
similar to this letter,  which letter or telex sets forth the material  terms of
the Transaction to which this Confirmation  relates and indicates your agreement
to those terms.

Yours sincerely,

SWISS BANK CORPORATION, LONDON BRANCH


By: /s/ Stewart Macbeth                  By: /s/ Karen Hayes
   ----------------------------             ------------------------
Name: Stewart Macbeth                    Name: Karen Hayes
Title: Director Fx & Derivatives         Title: Associate Director 
                                                SCBW London


Confirmed as of the 12th day
of May, 1997

INTERNEURON PHARMACEUTICALS, INC.



By:  /s/ Thomas F. Farb
Name: Thomas F. Farb
Title: Executive Vice President - Finance



                                       9





                          Appendix to Exhibit 10.90(b)*


The  Registrant  and Swiss Bank  Corporation,  London  Branch  entered  into two
confirmations,  relating to an aggregate of 2,000,000  options each  relating to
1,000,000 options and substantially  identical to the form of confirmation filed
as Exhibit  10.90(b),  except  that the  Expiration  Dates are May 21,  1999 for
contract B-1 and May 24, 1999 for contract B-2.


* Pursuant to section 2 to Item 601 of regulation S-K.