Exhibit 10.4


                                 LEUKOSITE, INC.


                          REGISTRATION RIGHTS AGREEMENT


     This REGISTRATION RIGHTS AGREEMENT (this "AGREEMENT") is made as of
July 20, 1999 by and among (i) LeukoSite, Inc., a Delaware corporation (the
"COMPANY"), (ii) HealthCare Ventures V, L.P., a Delaware limited partnership
(the "INITIAL INVESTOR"), and (iii) each person who becomes an Investor pursuant
to Section 10 hereof (together with the Initial Investor, the "INVESTORS" and
each individually, an "INVESTOR").

     WHEREAS, the Company has agreed to issue and sell to the Initial Investor,
and the Initial Investor has agreed to purchase from the Company, an aggregate
of 456,620 shares (the "SHARES") of the Company's common stock, $0.01 par value
per share (the "COMMON STOCK"), all upon the terms and conditions set forth in
the Stock Purchase Agreements, dated of even date herewith, between the Company
and the Initial Investor (the "STOCK PURCHASE AGREEMENTS"); and

     WHEREAS, the terms of the Stock Purchase Agreements provide that it shall
be a condition precedent to the closing of the transactions thereunder, for the
Company and the Initial Investor to execute and deliver this Agreement.

     NOW, THEREFORE, in consideration of the premises and mutual covenants
contained herein, the parties hereto hereby agree as follows:

     1. DEFINITIONS. The following terms shall have the meanings provided
therefor below or elsewhere in this Agreement as described below:

     "BOARD" shall mean the board of directors of the Company.

     "CLOSING DATE" shall mean the date on which the transactions contemplated
by the Stock Purchase Agreements are consummated (or, if the transactions are
not consummated on the same date, the first date by which the transactions
contemplated by the Stock Purchase Agreements have been consummated).

     "EXCHANGE ACT" shall mean the Securities Exchange Act of 1934, as amended.

     "MANDATORY REGISTRATION TERMINATION DATE" shall mean the first day after
the effective date of the Mandatory S-3 Registration Statement on which the
Mandatory S-3 Registration Statement (as defined in Section 2 hereof) shall not
be in effect.

     "PERSON" (whether or not capitalized) shall mean an individual,
partnership, limited liability company, corporation, association, trust, joint
venture, unincorporated


                                      -2-

organization, and any government, governmental department or agency or political
subdivision thereof.

     "QUALIFYING INVESTOR" shall have the meaning ascribed thereto in Section 11
hereof.

     "REGISTRABLE SHARES" shall mean, at the relevant time of reference thereto,
the Shares then held by the Investors (including any shares of capital stock
that were issued in respect thereof pursuant to a stock split, stock dividend,
recombination, reclassification or the like), PROVIDED, HOWEVER, that the term
"REGISTRABLE Shares" shall not include any of the Shares that (i) become
eligible for resale without volume limitations pursuant to Rule 144, or (ii) are
sold pursuant to a registration statement that has been declared effective under
the Securities Act by the SEC.

     "RULE 144" shall mean Rule 144 promulgated under the Securities Act and any
successor or substitute rule, law or provision.

     "SEC" shall mean the Securities and Exchange Commission.

     "SECURITIES ACT" shall mean the Securities Act of 1933, as amended.

     2. MANDATORY FORM S-3 REGISTRATION.

     (a) Within twenty (20) business days after the Closing Date, the Company
will prepare and file with the SEC a registration statement on Form S-3 for the
purpose of registering under the Securities Act all of the Registrable Shares
for resale by, and for the account of, the Initial Investor as the selling
stockholder thereunder (the "MANDATORY S-3 REGISTRATION STATEMENT"). The
Mandatory S-3 Registration Statement shall permit the Initial Investor to offer
and sell, on a delayed or continuous basis pursuant to Rule 415 under the
Securities Act, any or all of the Registrable Shares. The Company agrees to use
reasonable best efforts to cause the Mandatory S-3 Registration Statement to
become effective as soon as practicable. The Company shall only be required to
keep the Mandatory S-3 Registration Statement effective until the earlier to
occur of (i) the date when all of the Registrable Shares registered thereunder
shall have been sold and (ii) July 1, 2000. Thereafter, the Company shall be
entitled to withdraw the Mandatory S-3 Registration Statement and the Investors
shall have no further right to offer or sell any of the Registrable Shares
pursuant to the Mandatory S-3 Registration Statement (or any prospectus relating
thereto).

     (b) The Mandatory S-3 Registration Statement shall not be underwritten
unless the Company shall otherwise elect in its sole and absolute discretion.

     (c) Notwithstanding anything in this Section 2 to the contrary, if the
Company shall furnish to the Investors a certificate signed by the President or
Chief Executive Officer of the Company stating that the Board of Directors of
the Company has made the good faith determination (i) that continued use by the
Investors of the registration statement filed by the Company pursuant to this
Section 2 for purposes of effecting offers or sales of Registrable Shares
pursuant hereto would require, under the Securities Act and the rules and
regulations promulgated thereunder, premature


                                      -3-

disclosure in the registration statement (or the prospectus relating thereto) of
material, nonpublic information concerning the Company, its business or
prospects or any proposed material transaction involving the Company, (ii) that
such premature disclosure would be materially adverse to the Company, its
business or prospects or any such proposed material transaction or would make
the successful consummation by the Company of any such material transaction
significantly less likely and (iii) that it is therefore essential to suspend
the use by the Investors of such registration statement (and the prospectus
relating thereto) for purposes of effecting offers or sales of Registrable
Shares pursuant thereto, then the right of the Investors to use such
registration statement (and the prospectus relating thereto) for purposes of
effecting offers or sales of Registrable Shares pursuant thereto shall be
suspended for a period (the "SUSPENSION PERIOD") of not more than 90 days after
delivery by the Company of the certificate referred to above in this Section
2(c). During the Suspension Period, the Investors shall not offer or sell any
Registrable Shares pursuant to or in reliance upon such registration statement
(or the prospectus relating thereto). The Company agrees that, as promptly as
practicable after the consummation, abandonment or public disclosure of the
event or transaction that caused the Company to suspend the use of the
registration statement (and the prospectus relating thereto) pursuant to this
Section 2(c), the Company will provide the Investors with revised prospectuses,
if required, and will notify the Investors of their ability to effect offers or
sales of Registrable Shares pursuant to or in reliance upon such registration
statement.

     2A. DEMAND FORM S-3 REGISTRATION.

     (a) REGISTRATION UPON REQUEST; LIMITATIONS. In the event that, at any time
or from time to time after the Mandatory Registration Termination Date, the
Company shall receive from any Investor who holds at least twenty-five percent
(25%) of the Registrable Shares a written request or requests (a "DEMAND
NOTICE") that the Company effect a registration on Form S-3 (a "DEMAND
REGISTRATION"), or any successor or substitute form, with respect to all or a
part of the Registrable Shares owned by such Investor, then the Company will
promptly give written notice of the proposed registration and the Investor's or
Investors' request therefor to all other Investors, and, as soon as practicable,
use reasonable best efforts to effect such registration of all or such portion
of such Investor's or Registrable Shares as are specified in such request,
together with all or such portion of the Registrable Shares of any other
Investor or Investors joining in such request as are specified in a written
request given within ten (10) business days after receipt of such written notice
from the Company; PROVIDED, HOWEVER, that the Company's obligation under this
Section 2A(a) shall be temporarily suspended if the Company has previously given
a notice of the type specified in Section 3 hereof or this Section 2A(a)
("REGISTRATION NOTICE") from the date the Registration Notice is received until
the date the registration statement referred to in the Registration Notice is
declared effective (the "TEMPORARY SUSPENSION PERIOD"), so long as (i) the
Temporary Suspension Period is no longer than sixty-five (65) days, and (ii) the
Investors are informed in writing that the Company's obligation under this
Section 2A(a) have been temporarily suspended in accordance with this provision;
and PROVIDED, FURTHER, that the obligations of the Company under this Section
2A(a) shall be subject to the limitations set forth in Sections 2A(c), 2A(d) and
2A(e) below. The Company may include in any registration pursuant to Section
2A(a) hereof additional shares of Common Stock for sale for its own account or
for the account of any other person who has been granted piggy-back registration
rights. No registration under this Section 2A(a) shall


                                      -4-

be underwritten unless the Company shall otherwise elect in its sole and
absolute discretion.

     If the Company receives conflicting instructions, notices or elections from
two or more persons with respect to the same Registrable Shares, then the
Company may act upon the basis of the instructions, notice or election received
from the registered owner of such Registrable Shares.

     (b) SELECTION OF UNDERWRITERS. If a registration pursuant to Section 2A(a)
hereof involves an underwritten offering, the underwriter or underwriters
thereof shall be selected by the Company, provided that the underwriter or
underwriters so selected shall be a nationally recognized investment banking
firm or firms.

     (c) LIMITATION ON NUMBER OF REGISTRATIONS. The Company shall not be
required to effect (i) more than two (2) registrations pursuant to Section 2A(a)
and (ii) more than one registration pursuant to Section 2A(a) during any
consecutive nine (9) month period.

     (d) LIMITATION ON COMPANY'S OBLIGATION. Notwithstanding anything in this
Section 2A to the contrary, but in all events subject to the provisions of
Section 2A(f) hereof, the Company shall not be obligated to effect any
registration pursuant to Sections 2A and 3:

          (1) if Form S-3, or any successor or substitute form, is not then
available for the registration of such Registrable Shares proposed to be sold
and distributed by such Investor or Investors;

          (2) if such Investor or Investors, together with the holders of any
other securities of the Company entitled to inclusion in such registration,
propose to sell Registrable Shares and such other securities (if any) at an
aggregate price to the public of less than $750,000; or

          (3) if the Company shall furnish to the Investors a certificate signed
by the President and Chief Executive Officer of the Company stating that the
Board has made the good faith determination that a registration would require
premature disclosure of material, nonpublic information concerning the Company,
its business or prospects, that such premature disclosure would be materially
adverse to the Company and that it is therefore essential to suspend or defer
such registration, then the Company shall have the right either to suspend the
use of an effective registration statement or defer the filing of a registration
statement for a period of not more than ninety (90) days (the "DEFERRAL
PERIOD"); PROVIDED, HOWEVER, that the Company may not utilize this right more
than once with respect to each registration request (or registration statement
filed as a result of a request) made pursuant to, and in accordance with,
Section 2A(a) hereof. If the Board makes the determination described in the
preceding sentence, the Company shall give written notice of such determination
to the holders of Registrable Shares. The Company shall notify the holders of
the expiration of the Deferral Period and shall, if such registration statement
requested pursuant to Section 2A(a) hereof has not yet been filed, cause the
registration statement with respect to the Demand Registration to be filed on
the fifth (5th) business day following the expiration of the Deferral Period
(the "WITHDRAWAL PERIOD") (or, if registration on such date is not practicable,
as promptly as possible thereafter) unless, prior to the expiration of the
Withdrawal


                                      -5-

Period, the holders holding a majority of Registrable Shares to be
included in any such Demand Registration not yet filed, by written notice to the
Company, withdraws the request made under Section 2(a), in which case, such
request shall not count as one of the Demand Registrations permitted hereunder
and the Company shall pay all expenses in connection with such registration
theretofor incurred in accordance with Section 6 herein.

     (e) LIMITATION ON REQUESTS. Notwithstanding anything in this Section 2A to
the contrary, (1) no Investor may request a registration pursuant to this
Section 2A within one hundred and eighty (180) days of the effective date of any
other registration statement filed by the Company with the SEC pursuant to
Sections 2A and 3; and (2) no Investor may request a registration pursuant to
Section 2A(a) at any time after the seventh (7th) anniversary of the Closing
Date.

     (f) UNAVAILABILITY OF FORM S-3. Notwithstanding anything to the contrary
expressed or implied in this Agreement, if Form S-3 or any substitute form is
not then available for the registration of such Registrable Shares that would
otherwise have been proposed to be sold and distributed by such Investor or
Investors pursuant to this Section 2A, the Company shall be obligated to prepare
and file a registration statement on Form S-1 at the written request or requests
from any Investor or Investors given in accordance with Section 2A(a) and the
provisions of this Section 2A (other than Section 2A(d)(2)) shall govern and
apply to such request or requests and such registration on Form S-1.

     3.   "PIGGYBACK REGISTRATION".

          (a) If, at any time after the Mandatory Registration Termination Date,
the Company proposes to register any of its Common Stock under the Securities
Act, whether as a result of a primary or secondary offering of Common Stock or
pursuant to registration rights granted to holders of other securities of the
Company (but excluding in all cases any registration pursuant to Section 2A
hereof or any registrations to be effected on Forms S-4 or S-8 or other
applicable successor Forms), the Company shall, each such time, give to the
Investors twenty (20) days' prior written notice of its intent to do so, and
such notice shall describe the proposed registration and offer such holders the
opportunity to register such number of Registrable Shares as each such holder
may request. Upon the written request of any Investor given within ten (10) days
after the giving of any such notice by the Company, the Company shall use its
reasonable best efforts to cause to be included in such registration the
Registrable Shares of such selling Investor, to the extent requested to be
registered, among all holders of Registrable Shares and other persons entitled
to the inclusion of their shares in such registration, PRO RATA on the basis of
the number of shares of Common Stock that owned or held by such selling Investor
to all of the shares of Common Stock owned or held by all holders and other
persons entitled to be included within such registration; PROVIDED that (i) the
number of Registrable Shares proposed to be sold by such selling Investor is
equal to at least twenty-five percent (25%) of the total number of Registrable
Shares then held by such selling Investor, (ii) such selling Investor agrees to
sell those of its Registrable Shares to be included in such registration in the
same manner and on the same terms and conditions as the other shares of Common
Stock which the Company proposes to register, and (iii) if the registration is
to include shares of Common Stock to be sold for the account of the Company or
any party exercising demand registration


                                      -6-

rights pursuant to any other agreement with the Company, the proposed managing
underwriter does not advise the Company that in its opinion the inclusion of
such selling Registrable Shares (without any reduction in the number of shares
to be sold for the account of the Company or such party exercising demand
registration rights) is likely to affect materially and adversely the success of
the offering or the price that would be received for any shares of Common Stock
offered, in which case the rights of such selling Investor shall be as provided
in Section 3(b) hereof.

          (b) If a registration pursuant to Section 3 hereof involves an
underwritten offering and the managing underwriter shall advise the Company in
writing that, in its opinion, the number of shares of Common Stock requested by
the Investors to be included in such registration is likely to affect materially
and adversely the success of the offering or the price that would be received
for any shares of Common Stock offered in such offering, then, notwithstanding
anything in this Section 3 to the contrary, the Company shall only be required
to include in such registration, to the extent of the number of shares of Common
Stock which the Company is so advised can be sold in such offering, (i) first,
the number of shares of Common Stock proposed to be included in such
registration for the account of the Company and/or any stockholders of the
Company (other than the Investors) that have exercised demand registration
rights, in accordance with the priorities, if any, then existing among the
Company and/or such stockholders of the Company with registration rights (other
than the Investors), and (ii) second, the shares of Common Stock requested to be
included in such registration by all other stockholders of the Company
(including, without limitation, the Investors), PRO RATA among such other
stockholders (including, without limitation, the Investors) on the basis of the
number of shares of Common Stock that each of them requested to be included in
such registration.

          (c) In connection with any offering involving an underwriting of
shares, the Company shall not be required under this Section 3 or otherwise to
include the Registrable Shares of any Investor therein unless such Investor
accepts and agrees to the terms of the underwriting, which shall be reasonable
and customary, as agreed upon between the Company and the underwriters selected
by the Company.

     4. OBLIGATIONS OF THE COMPANY. Whenever the Company is required under
Sections 2A or 3 hereof to use its reasonable best efforts to effect the
registration of any of the Registrable Shares of the Investors, the Company
shall, as expeditiously as practicable:

          (a) Prepare and file with the SEC (not later than forty-five (45) days
after receipt of a request to file a registration statement with respect to
Registrable Shares pursuant to Section 3A hereof) a registration statement with
respect to such Registrable Shares and use its reasonable best efforts to cause
such registration statement to become and remain effective; PROVIDED, however
THAT, except to the extent otherwise provided in Section 2A hereof, the Company
shall in no event be obligated to cause any such registration to remain
effective for more than ninety (90) days; PROVIDED FURTHER, that before filing a
registration statement or prospectus or any amendments or supplements thereto,
the Company shall (i) use reasonable efforts to provide counsel selected by the
holders of a majority of the Registrable Shares being


                                      -7-

registered in such registration ("HOLDERS' COUNSEL") with an opportunity to
participate in the preparation of such registration statement and each
prospectus included therein (and each amendment or supplement thereto) to be
filed with the SEC, and (ii) notify the Holders' Counsel of any stop order
issued or threatened by the SEC and to take all reasonable action required to
prevent the entry of such stop order or to remove it if entered;

          (b) Prepare and file with the SEC such amendments and supplements to
such registration statement and the prospectus used in connection therewith as
may be necessary to comply with the provisions of the Securities Act with
respect to the disposition of all Registrable Shares covered by such
registration statement;

          (c) Notify the Investors and Holders' Counsel (if any) promptly and,
if requested by any Investor, confirm such advice in writing (i) when a
registration statement has become effective and when any post-effective
amendments and supplements thereto become effective, and (ii) of the issuance by
the SEC or any state securities commission of any stop order suspending the
effectiveness of a registration statement.

          (d) Furnish to the selling Investors such number of copies of a
prospectus, including a preliminary prospectus, in conformity with the
requirements of the Securities Act, and such other documents (including, without
limitation, prospectus amendments and supplements as are prepared by the Company
in accordance with Section 4(e) below) as the selling Investors may reasonably
request in order to facilitate the disposition of such Registrable Shares;

          (e) Notify the Investors and Holders' Counsel (if any), at any time
when a prospectus relating to such registration statement is required to be
delivered under the Securities Act, of the happening of any event as a result of
which the prospectus included in or relating to such registration statement
contains an untrue statement of a material fact or omits any fact necessary to
make the statements therein not misleading; and, thereafter, the Company will
promptly prepare (and, when completed, give notice to each Investor) a
supplement or amendment to such prospectus so that, as thereafter delivered to
the purchasers of such Registrable Shares, such prospectus will not contain an
untrue statement of a material fact or omit to state any fact necessary to make
the statements therein not misleading; PROVIDED that upon such notification by
the Company, the Investors will not offer or sell Registrable Shares until the
Company has notified the Investors that it has prepared a supplement or
amendment to such prospectus and delivered copies of such supplement or
amendment to the Investors (it being understood and agreed by the Company that
the foregoing proviso shall in no way diminish or otherwise impair the Company's
obligation to promptly prepare a prospectus amendment or supplement as above
provided in this Section 4(e) and deliver copies of same as above provided in
Section 4(d) hereof);

          (f) Use its reasonable best efforts to register and qualify such
Registrable Shares under such other securities or Blue Sky laws of such
jurisdictions as each selling Investor shall be reasonably request and do any
and all other acts or things which may be reasonably necessary or advisable to
enable each selling Investor to consummate the public sale or other disposition
in such jurisdiction of Registrable


                                      -8-

Shares, PROVIDED that the Company shall not be required in connection therewith
or as a condition thereto to qualify to do business or to file a general consent
to service of process in any such states or jurisdictions where it is not then
qualified or subject to process;

          (g) Use its reasonable best efforts to cause all Registrable Shares to
be listed on the Nasdaq National Stock Market; and

          (h) Make available for inspection by any seller of Registrable Shares,
any managing underwriter participating in any disposition pursuant to such
registration statement, Holders' Counsel (if any) and any attorney, accountant
or other agent retained by any such seller or any managing underwriter (each, an
"INSPECTOR" and collectively, the "INSPECTORS"), during regular business hours
and upon reasonable advance notice, all financial and other records, pertinent
corporate documents and properties of the Company (collectively, the "RECORDS")
as shall be reasonably necessary to enable them to exercise their due diligence
responsibility, and cause the Company's officers, directors and employees, and
the independent public accountants of the Company, to supply all information
reasonably requested by any such Inspector in connection with such registration
statement, subject to obligations of confidentiality.

     5. FURNISH INFORMATION. It shall be a condition precedent to the
obligations of the Company to take any action pursuant to this Agreement that
the selling Investors shall furnish to the Company such information regarding
them and the Shares held by them as the Company shall reasonably request and as
shall be required in order to effect any registration by the Company pursuant to
this Agreement.

     6. EXPENSES OF REGISTRATION. All expenses incurred in connection with a
registration pursuant to this Agreement (excluding underwriting commissions and
discounts of the selling Investors), including without limitation all
registration and qualification fees, printing expenses, and fees and
disbursements of counsel for the Company and one counsel for the selling
Investors, shall be borne by the Company.

     7. DELAY OF REGISTRATION. Subject to Section 13(d) hereof, the Investors
and the Company (other than with respect to Section 4(e)) shall not take any
action to restrain, enjoin or otherwise delay any registration as the result of
any controversy which might arise with respect to the interpretation or
implementation of this Agreement.

     8. INDEMNIFICATION. In the event that any Registrable Shares of the
Investors are included in a registration statement pursuant to this Agreement:

          (a) To the fullest extent permitted by law, the Company will indemnify
and hold harmless each selling Investor, any underwriter (as defined in the
Securities Act) for the Company, and each officer, director, fiduciary,
employee, member, general partner and limited partner (and affiliates thereof)
of such selling Investor or such underwriter, each broker or other person acting
on behalf of such selling Investor and each person, if any, who controls such
selling Investor or such underwriter within the meaning of the Securities Act,
against any losses, claims, damages or liabilities, joint


                                      -9-

or several, to which they may become subject under the Securities Act or
otherwise, insofar as such losses, claims, damages or liabilities (or actions in
respect thereof) arise out of or are based upon any untrue or alleged untrue
statement of any material fact contained in such registration statement,
including any preliminary prospectus or final prospectus contained therein or
any amendments or supplements thereto, or arise out of or are based upon the
omission or alleged omission to state therein a material fact required to be
stated therein, or necessary to make the statements therein not misleading, or
any violation by the Company of the Securities Act or state securities or blue
sky laws applicable to the Company and leading to action or inaction required of
the Company in connection with such registration or qualification under such
Securities Act or state securities or blue sky laws; and, subject to the
provisions of Section 8(c), the Company will reimburse on demand such selling
Investor, such underwriter, such broker or other person acting on behalf of such
selling Investor or such officer, director, fiduciary, employee, member, general
partner, limited partner, affiliate or controlling person for any legal or other
expenses reasonably incurred by any of them in connection with investigating or
defending any such loss, claim, damage, liability or action; PROVIDED, HOWEVER,
that the indemnity agreement contained in this Section 8(a) shall not apply to
amounts paid in settlement of any such loss, claim, damage, liability or action
if such settlement is effected without the consent of the Company (which consent
shall not be unreasonably withheld), nor shall the Company be liable in any such
case for any such loss, damage, liability or action to the extent that it arises
out of or is based upon an untrue statement or alleged untrue statement or
omission made in connection with such registration statement, preliminary
prospectus, final prospectus, or amendments or supplements thereto, in reliance
upon and in conformity with written information furnished expressly for use in
connection with such registration by the selling Investors, any underwriter for
them or controlling person with respect to them.

          (b) To the fullest extent permitted by law, each selling Investor will
indemnify and hold harmless the Company, each of its directors, each of its
officers who have signed such registration statement, each person, if any, who
controls the Company within the meaning of the Securities Act, any underwriter
for the Company (within the meaning of the Securities Act), and all other
selling Investors against any losses, claims, damages or liabilities to which
the Company or any such director, officer, controlling person, or underwriter
may become subject to, under the Securities Act or otherwise, insofar as such
losses, claims, damages or liabilities (or actions in respect thereto) arise out
of or are based upon any untrue or alleged untrue statement of any material fact
contained in such registration statement, including any preliminary prospectus
contained therein or any amendments or supplements thereto, or arise out of or
are based upon the omission or alleged omission to state therein a material fact
required to be stated therein or necessary to make the statements therein not
misleading, in each case to the extent that such untrue statement or alleged
untrue statement or omission or alleged omission was made in such registration
statement, preliminary prospectus, final prospectus, or amendments or
supplements thereto, in reliance upon and in conformity with written information
furnished by the selling Investor expressly for use in connection with such
registration; and, subject to the provisions of Section 8(c), such selling
Investor will reimburse on demand any legal or other expenses reasonably
incurred by the Company or any such director, officer, controlling person,
underwriter or other selling Investor in connection with


                                      -10-

investigating or defending any such loss, claim, damage, liability or action,
PROVIDED, HOWEVER, that the maximum amount of liability of each selling Investor
hereunder shall be limited to the proceeds (net of underwriting discounts and
commissions, if any) actually received by such selling Investor from the sale of
Registrable Shares covered by such registration statement; and PROVIDED,
FURTHER, HOWEVER, that the indemnity agreement contained in this Section 8(b)
shall not apply to amounts paid in settlement of any such loss, claim, damage,
liability or action if such settlement is effected without the consent of those
selling Investor(s) against which the request for indemnity is being made (which
consent shall not be unreasonably withheld).

          (c) Promptly after receipt by an indemnified party under this Section
8 of notice of the commencement of any action, such indemnified party will, if a
claim in respect thereof is to be made against any indemnifying party under this
Section 8, notify the indemnifying party in writing of the commencement thereof
and the indemnifying party shall have the right to participate in and, to the
extent the indemnifying party desires, jointly with any other indemnifying party
similarly noticed, to assume at its expense the defense thereof with counsel
mutually satisfactory to the parties; PROVIDED, HOWEVER, that, if any
indemnified party shall have reasonably concluded that there may be one or more
legal defenses available to such indemnified party which are different from or
additional to those available to the indemnifying party, or that such claim or
litigation involves or could have an effect upon matters beyond the scope of the
indemnity agreement provided in this Section 8, the indemnifying party shall not
have the right to assume the defense of such action on behalf of such
indemnified party, and such indemnifying party shall reimburse such indemnified
party and any person controlling such indemnified party for the fees and
expenses of counsel retained by the indemnified party which are reasonably
related to the matters covered by the indemnity agreement provided in this
Section 8. Subject to the foregoing, an indemnified party shall have the right
to employ separate counsel in any such action and to participate in the defense
thereof but the fees and expenses of such counsel shall not be at the expense of
the Company. The failure to notify an indemnifying party promptly of the
commencement of any such action, if materially prejudicial to his ability to
defend such action, shall relieve such indemnifying party of any liability to
the indemnified party under this Section 8, but the omission so to notify the
indemnifying party will not relieve him of any liability which he may have to
any indemnified party otherwise other than under this Section 8.

          (d) If the indemnification provided for in this Section 8 from the
indemnifying party is applicable by its terms but unavailable to an
indemnified party hereunder in respect of any losses, claims, damages,
liabilities or expenses referred to therein, then the indemnifying party, in
lieu of indemnifying such indemnified party, shall contribute to the amount
paid or payable by such indemnified party as a result of such losses, claims,
damages, liabilities or expenses in such proportion as is appropriate to
reflect the relative fault of the indemnifying party and indemnified party in
connection with the actions which resulted in such losses, claims, damages,
liabilities or expenses, as well as any other relevant equitable
considerations. The relative faults of such indemnifying party and
indemnified party shall be determined by reference to, among other things,
whether any action in question, including any untrue or alleged untrue
statement of a material fact or omission or alleged omission to state a
material fact, has been made by, or relates to information supplied by, such


                                      -11-


indemnifying party or indemnified party, and the parties' relative intent,
knowledge, access to information and opportunity to correct or prevent such
action. The amount paid or payable by a party as a result of the losses,
claims, damages, liabilities and expenses referred to above shall be deemed
to include, subject to the limitations set forth in Sections 8(a), 8(b) and
8(c), any legal or other fees, charges or expenses reasonably incurred by
such party in connection with any investigation or proceeding. No person
guilty of fraudulent misrepresentation (within the meaning of Section 11(f)
of the Securities Act) shall be entitled to contribution from any person. The
parties hereto agree that it would not be just and equitable if contribution
pursuant to this Section 8(d) were determined by pro rata allocation or by
any other method of allocation which does not take account of the equitable
considerations referred to in the immediately preceding paragraph.

          (e) Notwithstanding anything in this Section 8 to the contrary, if, in
connection with an underwritten public offering, the Company, the Investors and
the underwriters enter into an underwriting or purchase agreement relating to
such offering which contains provisions covering indemnification among the
parties, then the indemnification provision of this Section 8 shall be deemed
inoperative for purposes of such offering.

     9. REPORTS UNDER THE EXCHANGE ACT. With a view to making available to the
Investors the use of Section 2A hereof and the benefits of Rule 144 and any
other rule or regulation of the SEC that may at any time permit the Investors to
sell the Shares to the public without registration, the Company agrees to use
reasonable best efforts to: (i) make and keep public information available, as
those terms are understood and defined in the General Instructions to Form S-3,
or any successor or substitute form, and in Rule 144, (ii) file with the SEC in
a timely manner all reports and other documents required to be filed by an
issuer of securities registered under the Securities Act or the Exchange Act,
(iii) as long as any Investor owns any Shares, to furnish in writing upon such
Investor's request a written statement by the Company that it has complied with
the reporting requirements of Rule 144 and of the Securities Act and the
Exchange Act, and to furnish to such Investor a copy of the most recent annual
or quarterly report of the Company, and such other reports and documents so
filed by the Company as may be reasonably requested in availing such Investor of
any rule or regulation of the SEC permitting the selling of any such Shares
without registration and (iv) undertake any additional actions reasonably
necessary to maintain the availability of a registration statement on Form S-3,
including any successor or substitute forms, or the use of Rule 144.

     10. TRANSFER OF REGISTRATION RIGHTS. None of the rights of any Investor
under this Agreement shall be transferred or assigned to any person unless (i)
such person is a Qualifying Investor (as defined below), and (ii) such person
agrees to become a party to, and bound by, all of the terms and conditions of,
this Agreement. For purposes of this Section 10, the term "QUALIFYING INVESTOR"
shall mean, with respect to any Investor, (i) any partner, member or shareholder
thereof, (ii) any person, corporation or partnership controlling, controlled by,
or under common control with, such Investor or any partner thereof, or (iii) any
other direct transferee from such Investor of at least 257,000 shares of Common
Stock (subject to adjustment in the event of stock splits, stock dividends,
recombinations, recapitalizations and the like). None of


                                      -12-

the rights of any Investor under this Agreement shall be transferred or assigned
to any transferee of Shares pursuant to a "brokers transaction" within the
meaning of Rule 144 under the Securities Act or an effective registration
statement under the Securities Act. Upon transfer of Shares and rights in
accordance with this Section 10, such Qualified Investor shall be deemed an
"Investor" hereunder.

     11. LOCKUP AGREEMENT.

          (a) Each Investor which holds or owns (at the time of the written
request of the Company or managing underwriter referred to below in this Section
11(b) or at any time during the ninety (90) day period commencing on the
effective date of the registration statement relating to such underwriter public
offering of the Company's securities) of record or beneficially (within the
meaning of Section 13(d) of the Exchange Act and the rules and regulations
promulgated thereunder) five percent (5%) or more of the then issued and
outstanding shares of common stock of the Company (including, for this purpose,
shares held or owned by any and all affiliates of such Investor) hereby agrees
that, at the written request of the Company or any managing underwriter of any
underwritten public offering of securities of the Company, such Investor (and
its affiliates) shall not, without the prior written consent of the Company or
such managing underwriter, sell, make any short sale of, loan, grant any option
for the purchase of, pledge, encumber, or otherwise dispose of, or exercise any
registration rights with respect to, any Securities during the ninety (90) day
period commencing on the effective date of the registration statement relating
to such underwritten public offering of the Company's securities. If an Investor
is unable to sell, make any short sale of, loan, grant any option for the
purchase of, pledge, encumber, or otherwise dispose of, or exercise any
registration rights with respect to, any Securities pursuant to an effective
registration statement as a result of the foregoing sentence, and such effective
registration statement was filed pursuant to a request made under Section 2A
hereof, then such request shall not count as one of the Demand Registrations
permitted under Section 2A hereof.

          (b) The Company shall use reasonable best efforts to cause each
officer and director of the Company who holds or owns (at the time of the
written request of the Company or managing underwriter referred to below in this
Section 11 or at any time during the ninety (90) day period commencing on the
effective date of the registration statement relating to such underwriter public
offering of the Company's securities) of record or beneficially (within the
meaning of Section 13(d) of the Exchange Act and the rules and regulations
promulgated thereunder) five percent (5%) or more of the then issued and
outstanding shares of common stock of the Company, at the written request of the
Company or any managing underwriter of any underwritten public offering of
securities of the Company, not to, without the prior written consent of the
Company or such managing underwriter, sell, make any short sale of, loan, grant
any option for the purchase of, pledge, encumber, or otherwise dispose of, or
exercise any registration rights with respect to, any Securities during the
ninety (90) day period commencing on the effective date of the registration
statement relating to such underwritten public offering of the Company's
securities.

     12.  ENTIRE AGREEMENT. This Agreement constitutes and contains the entire
agreement and understanding of the parties with respect to the subject matter


                                      -13-

hereof, and it also supersedes any and all prior negotiations, correspondence,
agreements or understandings with respect to the subject matter hereof.

     13.  MISCELLANEOUS.

          (a) This Agreement may not be amended, modified or terminated, and no
rights or provisions may be waived, except with the written consent of the
Majority Holders and the Company.

          (b) This Agreement shall be governed by and construed and enforced in
accordance with the laws of the Commonwealth of Massachusetts, and shall be
binding upon the parties hereto and their respective heirs, personal
representatives, successors and permitted assigns and transferees, PROVIDED that
the terms and conditions of Section 10 hereof are satisfied. Notwithstanding
anything in this Agreement to the contrary, if at any time any Investor shall
cease to own any Shares, all of such Investor's rights under this Agreement
shall immediately terminate.

          (c) Any notices to be given pursuant to this Agreement shall be in
writing and shall be given by certified or registered mail, return receipt
request. Notices shall be deemed given when personally delivered or when mailed
to the addresses of the respective parties as set forth on EXHIBIT A hereto, or
to such changed address of which any party may notify the others pursuant
hereto, except that a notice of change of address shall be deemed given when
received.

          (d) The parties acknowledge and agree that in the event of any breach
of this Agreement, remedies at law will be inadequate, and each of the parties
hereto shall be entitled to specific performance of the obligations of the other
parties hereto and to such appropriate injunctive relief as may be granted by a
court of competent jurisdiction. All remedies, either under this Agreement or by
law or otherwise afforded to any of the parties, shall be cumulative and not
alternative.

          (e) This Agreement may be executed in a number of counterparts. All
such counterparts together shall constitute one Agreement, and shall be binding
on all the parties hereto notwithstanding that all such parties have not signed
the same counterpart. The parties hereto confirm that any facsimile copy of
another party's executed counterpart of this Agreement (or its signature page
thereof) will be deemed to be an executed original thereof.

          (f) Except as contemplated in Section 8 hereof, this Agreement is
intended solely for the benefit of the parties hereto and is not intended to
confer any benefits upon, or create any rights in favor of, any Person
(including, without limitation, any stockholder or debt holder of the Company)
other than the parties hereto.

          (g) If any provision of this Agreement is invalid, illegal or
unenforceable, such provision shall be ineffective to the extent, but only to
the extent of, such invalidity, illegality or unenforceability, without
invalidating the remainder of such provision or the remaining provisions of this
Agreement, unless such a construction would be unreasonable.


                                      -14-

          (h) This Agreement shall be binding upon, and inure to the benefit of,
the parties hereto and their permitted successors and assigns.



                          [ Signature Page to follow ]






     IN WITNESS WHEREOF, the parties hereto have executed this Registration
          Rights Agreement as of the date and year first above written.

                                 LEUKOSITE, INC.



                                 By: /s/ Christopher K. Mirabelli
                                    -----------------------------
                                 Name:   Christopher K. Mirabelli
                                 Title:  President and CEO

                                 INITIAL INVESTOR:

                                 HEALTHCARE VENTURES V, L.P.
                                 By:


                                 By: /s/ Jeffrey Steinberg
                                    -----------------------------
                                 Name:   Jeffrey Steinberg
                                 Title:  Administrative Partner of
                                         HealthCare Partner V, L.P., The
                                         General Partner of HealthCare
                                         Vartives V, L.P.






                                    EXHIBIT A

    All correspondence to the Company shall be addressed as follows:

               LeukoSite, Inc.
               215 First Street
               Cambridge, MA 02142
               Attention: Christopher K. Mirabelli,
               President and Chief Executive Officer
               Telecopier:  (617) 278-3399

    with a copy to:

               Bingham Dana LLP
               150 Federal Street
               Boston, Massachusetts 02110
               Attention: Julio E. Vega, Esq.
               Telecopier: (617) 951-8736

    All correspondence to the Initial Investor shall be addressed as follows:

               HealthCare Ventures V, L.P.
               44 Nassau Street
               Princeton, NJ  08542-4511
               Telecopy:   (609) 430-9525
               Attention:  Jeffrey Steinberg