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Attachment B

Registration Rights Agreement

June 27, 2000

To:  Toray Industries, Inc. (the "Investor")

Dear Sirs:

      In connection with the issuance to you of a Stock Option Grant of 500,000
shares of Common Stock $.01 par value ("Common Shares"), of United Therapeutics
Corporation, a Delaware corporation ("the Company"), pursuant to the Stock
Option Grant dated June 27, 2000 between the Company and the Investor (the
"Stock Option Grant"), the Company hereby covenants and agrees with you, and
with each subsequent holder of Restricted Stock (as such term is defined
herein), as follows:

      1. Certain Definitions.  As used herein, the following terms shall have
the following respective meanings:

            "Commission" shall mean the Securities and Exchange Commission, or
any other federal agency at the time administering the Securities Act.

            "Common Shares" shall mean the 500,000 shares of Common Stock to
which Toray has the right to acquire in accordance with the Stock Option Grant.

            "Exchange Act" shall mean the Securities Exchange Act of 1934 or
any similar federal statute, and the rules and regulations of the Commission
thereunder, all as the same shall be in effect at the time.

            "Public Sale" shall mean any sale of Common Stock to the public
pursuant to an offering registered under the Securities Act or to the public
pursuant to the provisions of Ruth 144 (or any successor or similar rule)
adopted under the Securities Act.

            "Registration Expenses" shall mean the expenses so described in
Section 6 hereof.

            "Restricted Stock" shall mean, subject to the provisions of Section
8 hereof, (i) the Common Shares and (ii) any securities issued as a dividend or
other distribution with respect to or in exchange for or in replacement of such
Common Shares, the certificates for which are required by the provisions of
this Agreement and applicable law to bear the legend set forth in the Section.

            "Securities Act" shall mean the Securities Act of 1933 or any
similar federal statute, and the rules and regulations of the Commission
thereunder, all as the same shall be in effect at the time.

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            "Selling Expenses" shall mean the expenses so described in Section
6 hereof.

      2. Restrictive Legend and Lock-Up Agreement. Each certificate
representing the Common Shares and, other than in a Public Sale or as otherwise
provided in Section 3 hereof, each certificate issued upon exchange or transfer
of any Common Shares, shall be stamped or otherwise imprinted with a legend
substantially in the following form:

"The securities represented by this certificate have not been registered or
qualified under the Federal Securities Act of 1933 (the "1933 Act") or
applicable state securities laws and are "restricted securities" within the
meaning of Rule 144 promulgated under the 1933 Act. The securities may not be
sold or transferred without complying with Rule 144 in the absence of effective
registration under the 1933 Act or other compliance under or exemption from the
1933 Act and applicable state securities laws."

      3. Notice of Proposed Transfer. Prior to any proposed transfer of any
Restricted Stock (other than under the circumstances described in Section 4
hereof), the holder thereof shall give written notice to the Company of its
intention to effect such transfer. Each such notice shall describe the manner
of the proposed transfer and, if requested by the Company, shall be accompanied
by an opinion of counsel reasonably satisfactory to the Company to the effect
that the proposed transfer of such Restricted Stock may be effected without
registration under the Securities Act, whereupon the holder of such Restricted
Stock shall be entitled to transfer such Restricted Stock in accordance with
the terms of its notice; provided, however, that in the case of any Investor
that is a partnership, no such opinion or other documentation shall be required
if such notice shall cover a transfer by such partnership to its partners and
provided, further, however, that the shares so transferred shall remain subject
to this Agreement. Each certificate representing the Restricted Stock
transferred as above provided shall bear the legend set forth in Section 2,
unless (i) such transfer is in accordance with the provisions of Rule 144 (or
any other rule permitting public sale without registration under the Securities
Act) and is not made by an affiliate of the Company or (ii) the opinion of
counsel referred to above is to the further effect that the transferee and any
subsequent transferee (other than an affiliate of the Company) would be
entitled to transfer such securities without registration under the Securities
Act.

      The foregoing restrictions on transferability of Restricted Stock shall
terminate as to any particular shares of Restricted Stock when such shares
shall have been effectively registered under the Securities Act and sold or
otherwise disposed of in accordance with the intended method of disposition by
the seller or the sellers thereof set forth in the registration statement
concerning such shares. Whenever a holder of Restricted Stock is able to
demonstrate to the Company (and its counsel) that the provisions of Rule 144(k)
of the Securities Act are available to such holder without limitation, such
holder of Restricted Stock shall be entitled to receive from the Company,
without expense, a new certificate not bearing the restrictive legend set forth
in Section 2 in exchange for the surrender of the existing certificate, which
shall be marked canceled by the Company.

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      4. Incidental Registration.

      a.    If the Company at any time following the date on which the Investor
may first exercise its stock options pursuant to the Stock Option Grant
proposes to register any of its Common Stock under the Securities Act for sale
to the public, whether for its own account or for the account of other security
holders or both (except with respect to registration statements on Form S-4 or
S-8 or another form not available for registering the Restricted Stock for sale
to the public), it will give written notice at such time to all holders of
outstanding Restricted Stock of its intention to do so. Upon the written
request of any such holder, given within 10 days after receipt of any such
notice by the Company, to register any of its Restricted Stock (which request
shall state the intended method of disposition thereof), the Company will use
its best efforts to cause the Restricted Stock as to which registration shall
have been so requested to be included in the securities to be covered by the
registration statement proposed to be filed by the Company, all to the extent
requisite to permit the sale or other disposition by the holder in accordance
with its written request) of such Restricted Stock. In the event that any
registration pursuant to this Section 4 shall be, in whole or in part, an
underwritten public offering of Common Stock (other than the Company's initial
public offering), then in such event the Company shall specify that either (i)
such Restricted Stock is to be included in the underwriting on the same terms
and conditions as the shares of Common Stock otherwise being sold through
underwriters under such registration, or (ii) such Restricted Stock is to be
sold in the open marker without any underwriting, on terms and conditions
comparable to those normally applicable to offerings of common stock in
reasonably similar circumstances. In any such underwritten public offering of
Common Stock, if, in the opinion of the managing underwriter, the Restricted
Stock so requested to be registered would adversely affect the marketing of
such Common Stock, the Company shall include in such registration, to the
extent of the number which the Company is advised can be sold in such offering,
(x) first, securities proposed by the Company to be sold for its own account,
(y) second, Restricted Stock requested to be included in such registration by
the holder or holders thereof and other securities of the Company requested to
be included in such registration pursuant to registration rights granted by the
Company to the holders of such securities prior to the date hereof, pro rata
among the requesting holders of Restricted Stock and such other securities
based upon the number of shares of Restricted Stock and such other Securities
requested to be registered, and (z) third, other securities of the Company
requested to be included in such registration (other than as described in
clause (y) above).

      b.    In the event that the Common Shares have not been registered by the
Company in accordance with Section 4(a) above within six (6) months following
the date the Investor may first exercise its stock options pursuant to the
Stock Option Grant, the Company shall register the Common Shares under the
Securities Act for sale to the public within fifteen (15) business days
thereafter.

      5. Registration Procedures. If and whenever the Company is required by
the provisions of Section 4 hereof to use its reasonable best efforts to effect
the registration of any of the Restricted Stock under the Securities Act, the
Company will, as expeditiously as possible:

            a.    prepare (and afford counsel for the selling holders
reasonable opportunity to review and comment thereon) and file with the
Commission a registration statement with

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respect to such securities and use its best efforts to cause such registration
statement to become and remain effective for the period of the distribution
contemplated thereby (determined as hereinafter provided);

            b.    prepare (and afford counsel for the selling holders
reasonable opportunity to review and comment thereon) and file with the
Commission such amendments and supplements to such registration statement and
the prospectus used in connection therewith as may be necessary to keep such
registration statement effective for the period specified in paragraph 5(a)
above and as shall comply with the provisions of the Securities Act with
respect to the disposition of all Restricted Stock covered by such registration
statement for such period;

            c.    furnish to each seller and to each underwriter such number of
copies of the registration statement and the prospectus included therein
(including each preliminary prospectus) as such persons may reasonably request
in order to facilitate the public sale or other disposition of the Restricted
Stock covered by such registration statement;

            d.    use its best efforts to register or qualify the Registered
Stock covered by such registration statement under the securities or blue sky
laws of such jurisdictions as the sellers of Restricted Stock or, in the case
of an underwritten public offering, the managing underwriter, shall reasonably
request, and use its best efforts to list all Restricted Stock covered by such
registration statement on any securities exchange on which any other securities
exchange on which any other securities of the same class as the Restricted
Stock are then listed;

            e.    immediately notify each seller under such registration
statement and each underwriter, at any time when a prospectus relating thereto
is required to be delivered under the Securities Act, of the happening of any
even as a result of which the prospectus contained in such registration
statement, as then in effect, includes an untrue statement of a material fact
or omits to state any material fact required to be stated therein or necessary
to make the statements therein now misleading in the light of the circumstances
then exiting;

            f.    use its best efforts (if the offering is underwritten) to
furnish, at the request of any seller, on the date that Restricted Stock is
delivered to the underwriters for sale pursuant to such registration: (i) an
opinion dated such date of counsel representing the Company for the purposes of
such registration, addressed to the underwriters and to such seller, stating
that such registration statement has become effective under the Securities Act
and that (A) to the knowledge of such counsel, no stop order suspending the
effectiveness thereof has been issued and no proceedings for that purpose have
been instituted or are pending or contemplated under the Securities Act, (B)
the registration statement, the related prospectus, and each amendment or
supplement thereof, comply as to form in all material respects with the
requirements of the Securities Act and the applicable rules and regulations of
the Commission thereunder (except that such counsel need express no opinion as
to financial statements contained therein or any information provided by the
underwriters or the sellers) and (C) to such other effects as may reasonably be
requested by counsel for the underwriters, and (ii) a letter dated such date
from the independent public accountants retained by the Company, addressed to
the underwriters and to such seller, stating that they are independent public
accountants within the meaning of the Securities Act and that, in the opinion
of such accountants, the financial statements of the

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Company included in the registration statement or the prospectus, or any
amendment or supplement thereof, comply as to form in all material respects
with the applicable accounting requirements of the Securities Act, and such
letter shall additionally cover such other financial matters (including
information as to the period ending no more than five business days prior to
the date of such letter) with respect to the registration in respect of which
such letter is being given as such underwriters or seller may reasonably
request; and

            g.    make available for inspection by each seller, any underwriter
participating in any distribution pursuant to such registration statement, and
any attorney, accountant or other agent retained by such seller or underwriter,
all financial and other records, pertinent corporate documents and properties
of the company, and cause the Company's officers, directors and employees to
supply all information reasonably requested by any such seller, underwriter,
attorney, accountant or agent in connection with such registration statement
and permit such seller, attorney, accountant or agent to participate in the
preparation of such registration statement.

For purposes of paragraphs (a) and (b) above, the period of distribution of
Restricted Stock in a firm commitment underwritten public offering shall be
deemed to extend until each underwriter has completed the distribution of all
securities purchased by it, and the period of distribution of Restricted Stock
in any other registration shall be deemed to extend until the earlier of the
sale of all Restricted Stock covered thereby or six months after the effective
date thereof.

      In connection with each registration hereunder, the selling holders of
Restricted Stock will furnish to the Company in writing such information with
respect to themselves and the proposed distribution by them as shall be
reasonably necessary in order to assure compliance with federal and applicable
state securities laws.

      In connection with each registration pursuant to Section 4 hereof
covering an underwritten public offering, the company agrees to enter into a
written agreement with the managing underwriter selected in the manner herein
provided in such form and containing such provisions as are customary in the
securities business for such an arrangement between underwriters and companies
of the Company's size and investment stature, provided that such agreement
shall not contain any such provision applicable to the Company which is
inconsistent with the provisions hereof and provided, further, that the time
and place of the closing under said agreement shall be as mutually agreed upon
among the Company and such underwriter.

      6.    Expenses. All expenses incurred by the Company in complying with
Section 4 hereof, including, without limitation, all registration and filing
fees, printing expenses, fees and disbursements of counsel and independent
public accountants for the Company, fees of the National Association of
Securities Dealers, Inc. or any successor thereto, transfer taxes, fees or
transfer agents and registrars, and costs of insurance, but excluding any
Selling Expenses, are herein called "Registration Expenses". All underwriting
discounts and selling commissions applicable to the sale of Restricted Stock
are herein called "Selling Expenses".

      The Company will pay all Registration Expenses in connection with each
registration statement filed pursuant to Section 4 hereof. All Selling Expenses
in connection with any

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registration statement filed pursuant to Section 4 hereof shall be borne by the
participating sellers in proportion to the number of shares sold by each, or by
such persons other than the Company (except to the extent the Company shall be
a seller) as they may agree.

      7. Representations and Warranties of the Company. The Company represents
and warrants to you as follows:

            a.    The execution, delivery and performance of this Agreement by
the Company have been duly authorized by all requisite corporate action and
will not violate any provision of law, any order of any court or other agency
of government, the Certificate of Incorporation or By-laws of the Company, or
any provision of any indenture, agreement or other instrument to which it or
any of its properties or assets is bound, or conflict with, result in a breach
of or constitute (with due notice or lapse of time or both) a default under any
such indenture, agreement or other instrument, or result in the creation or
imposition of any lien, charge or encumbrance of any nature whatsoever upon any
of the properties or assets of the Company.

            b.    This Agreement has been duly executed and delivered by the
Company and constitutes the legal, valid and binding obligation of the Company,
enforceable in accordance with its terms, subject to applicable bankruptcy,
insolvency, reorganization, moratorium and other similar laws from time to time
in effect affecting the enforcement of creditors rights generally and to
general equitable principles.

      8. Rule 144 Reporting. The Company agrees with you as follows:

            a.    The Company shall make and keep public information available,
as those terms are understood and defined in Rule 144 under the Securities Act
(or any successor act, regulation or rule thereto), at all times from and after
the date it is first required to do so.

            b.    The Company shall file with the Commission in a timely manner
all reports and other documents as the Commission may prescribe under Section
13(a) or 15(d) of the Exchange Act (or any successor act, regulation or rule
thereto) at any time after the Company has become subject to such reporting
requirements of the Exchange Act.

            c.    The Company shall furnish to such holder of Restricted Stock
forthwith upon request (i) a written statement by the Company as to its
compliance with the reporting requirements of Rule 144 (at any time from and
after the date it first becomes subject to such reporting requirements), and of
the Securities Act and the Exchange Act (at any time after it has become
subject to such reporting requirements), (ii) a copy of the most recent annual
or quarterly report of the Company, and (iii) such other reports and documents
so filed as a holder may reasonably request to avail itself of any rule or
regulation of the Commission allowing a holder of Restricted Stock to sell any
such securities without registration.

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      9.    Miscellaneous.

            a.    All covenants and agreements contained in this Agreement by
or on behalf of any of the parties hereto shall bind and inure to the benefit
of the respective successors and assigns of the parties hereto whether so
expressed or not. Without limiting the generality of the foregoing, the
registration rights conferred herein on the holders of Restricted Stock shall
inure to the benefit of any and all subsequent holders from time to time of the
Restricted Stock for so long as the certificates representing the Restricted
Stock shall be required to bear the legend specified in Section 2 hereof.

            b.    This Agreement shall be governed by and construed in
accordance with the law of the State of Delaware.

            c.    No change, modification, extension, termination or waiver of
this Agreement, or any of the provisions herein contained, shall be valid
unless made in writing and signed by duly authorized representatives of the
parties

            d.    If any provision of this Agreement is found to be invalid or
unenforceable in any jurisdiction, it shall be ineffective to the extent of
such invalidity or unenforceability in such jurisdiction, without rendering
invalid or unenforceable the remaining provisions hereof and without affecting
the validity or unenforceability of any of the terms of this Agreement in any
other jurisdiction. The parties agree to renegotiate in good faith any term
held invalid and be bound by the mutually agreed substitute provision.

            Please indicate your acceptance of the forgoing by signing and
returning the enclosed counterpart of this letter, whereupon this letter
(herein sometimes called "this Agreement") shall be a binding agreement between
the Company any you.

                                    Very truly yours,

                                    UNITED THERAPEUTICS CORPORATION

                                    /s/ Martine A. Rothblatt
                                    --------------------------------
                                    Martine A. Rothblatt
                                    Chief Executive Officer

AGREED TO AND ACCEPTED as of the date first above written:

                                    TORAY INDUSTRIES, INC.

                                    /s/Kiyoteru Wakasugi
                                    --------------------------------
                                    Kiyoteru Wakasugi
                                    Managing Director of the Board