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THIS STOCK OPTION GRANT, AND ANY SECURITIES WHICH MAY BE ACQUIRED UPON THE
EXERCISE OF THIS STOCK OPTION GRANT, HAVE NOT BEEN REGISTERED UNDER THE
SECURITIES ACT OF 1933, AS AMENDED, AND MAY NOT BE SOLD, HYPOTHECATED OR
OTHERWISE TRANSFERRED OR DISPOSED OF IN THE ABSENCE OF SUCH REGISTRATION,
UNLESS AN EXEMPTION FROM THE REQUIREMENT OF SUCH REGISTRATION IS AVAILABLE
UNDER THE CIRCUMSTANCES AT THE TIME OBTAINING (AND, IF REASONABLY REQUESTED BY
THE COMPANY, DEMONSTRATED BY AN OPINION OF COUNSEL SATISFACTORY TO THE
COMPANY).

STOCK OPTION GRANT TO PURCHASE
500,000 SHARES OF THE COMMON PAR VALUE $0.01 PER SHARE,
OF UNITED THERAPEUTICS CORPORATION

      For value received, Toray Industries, Inc., a Japan corporation, or its
permitted successors or assigns (collectively, the "Investor"), shall have the
option to subscribe for and purchase from UNITED THERAPEUTICS CORPORATION, a
Delaware corporation (the "Company"), up to 500,000 fully paid and
nonassessable shares (the "Shares") of the Company's common stock, par value
$0.01 per share ("Common Stock"), at the purchase price equivalent to the
averaged NASDAQ closing price during one calendar month ending on the date when
the Investor makes the first delivery to the Company of the clinical trial
materials with all reasonably required documentation concerned as required
under that Exclusive License Agreement dated as of June 23, 2000 (the "Exercise
Price"), exercisable by the Investor at any time from the date the Investor
makes the first delivery to the Company of the clinical trial materials with
the documentation concerned until that date thirty (30) days after FDA approval
of the Product in accordance with that Exclusive License Agreement dated as of
June 23, 2000.

The number and character of Shares issuable pursuant to this Stock Option Grant
are subject to adjustment as provided herein.

      This Stock Option Grant is subject to the following additional
provisions, terms and conditions:

      1.    Exercise of Stock Option Grant.

            a.    Manner of Exercise. This Stock Option Grant may be exercised
by the holder hereof, in whole or in part, by surrender of this Stock Option
Grant, together with written notice of exercise (a sample form of exercise is
attached hereto as Attachment A), to the Company at the principal office of the
Company (which, for the purposes of this Stock Option Grant, shall be deemed to
be the Company's address for notice purposes as provided in Section 8 below)
accompanied by payment (in cash, certified check bank draft or wire transfer
payable to the order of the Company) of the Exercise Price multiplied by the
number of Shares for which

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this Stock Option Grant is being exercised, or by exercise of the Net Exercise
Right as provided in Section 1(b) below.

            b.    Net Exercise Right. In lieu of payment of the Exercise price
pursuant to Section 1(a) above, this Stock Option Grant may also be exercised,
in whole or in part, by surrender of this Stock Option Grant together with
written notice of exercise specifying the holder's election to convert all, or
any specified portion, of this Stock Option Grant (the "Net Exercise Right")
into the number of Shares equal to the quotient obtained by dividing:

      (x) the value of the Shares for which the Stock Option Grant is then
being exercised (determined by subtracting the aggregate Exercise Price of such
Shares in effect immediately prior to the exercise of the Net Exercise Right
from the aggregate fair market value of the Shares immediately prior to the
exercise of the Net Exercise Right) by

      (y) the fair market value of one Share immediately prior to the exercise
of the Net Exercise Right.

The fair market value of a Share as of a particular date shall be determined as
follows:

                  (1)   If the Common Stock is publicly traded on a particular
valuation date, fair market value on such date shall be:

      (i) the closing sale price of the Common Stock, as quoted on any national
securities exchange on which stock shall be listed and registered, on the
business day immediately preceding the valuation date;

      (ii) the closing sale price of the Common Stock, if such stock is then
quoted on the National Association of Securities Dealers, Inc. ("NASDAQ")
National Market, on the business day immediately preceding the valuation date;
or

      (iii) if the Common Stock is not then traded on a national securities
exchange or on the NASDAQ National Market, but is quoted on NASDAQ, the average
of the closing bid and asked prices for the Common Stock as reported on NASDAQ
on the business day immediately preceding the valuation date.

                  (2)   If the Common stock is not publicly traded on a
particular valuation date, fair market value on such date shall be determined
by the Board of Directors of the Company acting in good faith (taking into
account, as such Board of Directors acting in good faith deems appropriate, any
recent corporate events involving a determination of value for the Company's
securities (e.g., the closing of an offering of securities or the granting of
incentive stock options)).

            c.    When Exercise Effective. Each exercise of this Stock Option
Grant shall be deemed to have been effected on the date on which the Investor
provides the Company with the deliveries contemplated by Section 1(a) above.

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            d.    Delivery of Stock Certificates. Certificates representing the
Shares purchased upon any exercise of this Stock Option Grant shall bear the
restrictive legend set forth in Section 9 below and shall be delivered to the
Investor promptly following such exercise. A new Stock Option Grant exercisable
for the number of Shares, if any, with respect to which this Stock Option Grant
shall not have been exercised shall also be delivered to the Investor. No
fractional Shares shall be issued upon the exercise of this Stock Option Grant;
rather, the Investor shall receive an amount, in cash, equal to the fair market
value of any such fractional Shares (to be determined with reference to the
provisions of Section 1(b) above).

      2.    Shares Issuable Upon Exercise.

            a.    Reserved. The Company covenants and agrees that all Shares
that may be issued upon the exercise of this Stock Option Grant shall, upon
issuance pursuant to an exercise of this Stock Option Grant in accordance with
its terms, be duly authorized and validly issued, fully paid and nonassessable
Shares, free and clear of all preemptive rights. The Company will at all times
reserve and keep available, solely for issuance and delivery upon the exercise
of this Stock Option Grant, such number Shares of the Common Stock as shall be
issuable from time to time upon the exercise of this Stock Option Grant.

            b.    Restricted Securities. Investor acknowledges that the Company
is issuing this Stock Option Grant upon the exemption from registration
provided in Section 4(2) of the Securities Act and is relying upon Investor's
representation that it is an "accredited investor" within the meaning of Rule
501 of Regulation D under the Securities Act of 1933, as amended. Until the
Shares that may be issued upon the exercise of this Stock Option Grant may be
registered in accordance with the Registration Rights Agreement that is
attached hereto at Attachment B and incorporated herein, Investor acknowledges
that the Shares to be issued upon the exercise of this Stock Option Grant may
be unregistered at the time of exercise and, in such event, Investor agrees
that the Shares be issued upon the exercise of this Stock Option Grant may only
be pledged, offered, sold or transferred if registered under the Securities Act
or pursuant to an exemption from the registration requirements thereunder.
Investor understands that absent registration of the Shares to be issued upon
the exercise of this Stock Option Grant under the Securities Act in accordance
with the Registration Rights Agreement at Attachment B, compliance with an
applicable exemption under the Securities Act is required for a sale or other
disposition of such shares.

      3.    Adjustment. The Exercise Price and/or the number and type of
securities issuable upon any exercise of this Stock Option Grant shall be
subject from time to time as hereinafter provided in Section 3.

            a.    If the Company, at any time, divides the outstanding shares
of its Common Stock into a greater number of shares (whether pursuant to a
stock split, stock dividend or otherwise) and conversely, if the outstanding
shares of its Common Stock are combined into a smaller number of shares, the
number of Shares available upon any exercise of this Stock Option Grant and the
Exercise Price in effect immediately prior to such division or combination
shall be proportionately adjusted to reflect the reduction or increase in
outstanding shares.

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            b.    If any capital reorganization or reclassification of the
capital stock of the Company, or consolidation or merger of the Company with
another corporation, or the sale of all or substantially all of its assets to
another corporation shall be effected in such a way that holders of shares of
Common Stock shall be entitled to receive stock, other securities or assets
with respect to or in exchange for such shares of Common Stock, then, as a
condition of such reorganization, reclassification, consolidation, merger or
sale, the holder of this Stock Option Grant shall have the right to purchase
and receive upon the basis and upon the terms and conditions specified in this
Stock Option Grant and in lieu of the Shares immediately theretofore
purchasable and receivable upon the exercise upon the exercise of this Stock
Option Grant, such shares of stock, other securities or assets as would have
been issued or delivered to the Investor if the Investor had exercised this
Stock Option Grant and had received such Shares prior to such reorganization,
reclassification, consolidation, merger or sale. The Company shall not effect
any such consolidation or merger unless, prior to the consummation thereof, the
successor corporation (if other than the Company) resulting from such
consolidation or merger shall assume by written instrument executed and mailed
to the Investor (i) the obligation to deliver to the Investor such shares of
stock, other securities or assets as, in accordance with the foregoing
provisions, the Investor may be entitled to purchase and (ii) the other
obligations of the Company set forth or referred to in this Stock Option Grant.

            c.    If, after the initial issuance of this Stock Option Grant to
the Investor, the Company shall declare a dividend or distribution payable to
holders of the Common Stock (whether payable in cash, securities or other
assets of the Company), upon any exercise of this Stock Option Grant the
Investor shall be entitled to receive, and the Company shall promptly pay to
the Investor, any such dividend(s) and/or distributions(s) (as well as any
other cash, securities or other assets which the Investor would have received
had it held any securities received in any such dividend(s) and/or
distributions(s)), also giving effect to the other provisions of this Section
3.

            d.    Promptly following any adjustment under this Section 3, the
Company shall give written notice thereof (by first class mail, postage
prepaid) to the Investor (at the Investor's address as shown on the books of
the Company), which notice shall state the Exercise Price and number of Shares
(or other securities or assets) resulting from such adjustment, setting forth
in reasonable detail the method of calculation and the facts upon which such
calculation is based.

            e.    The terms of this Stock Option Grant shall be binding upon
the successors of the Company.

      4.    No Rights as Stockholder; Notice. The Investor shall not by virtue
of this Stock Option Grant be entitled to any rights of a stockholder of the
Company. However, the Company shall give notice to the Investor if at any time
prior to the expiration or exercise in full of this Stock Option Grant any of
the following events shall occur:

      (a)   the Company shall declare any dividend or distribution with respect
to its capital stock;

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      (b)   a dissolution, liquidation or winding up of the Company shall be
proposed; or

      (c)   a capital reorganization or reclassification of the capital stock
of the Company, any consolidation or merger of the Company with or into another
corporation, any transaction or series of transactions in which more than fifty
percent (50%) of the voting securities of the Company are transferred to
another person, or any sale or conveyance to another corporation of the
property of the Company as an entirety or substantially as an entirety.

Such giving of notice shall be effected at least fifteen (15) business days
prior to the date fixed as a record date or effective date or the date of
closing of the Company's stock transfer books for the determination of the
stockholders entitled to such dividend or distribution, or for the
determination of the stockholders entitled to vote on such proposed merger,
consolidation, sale, conveyance, dissolution, liquidation or winding up. Such
notice shall specify such record date or the closing date or the date of
closing the stock transfer books, as the case may be.

      5.    Registered Owner. The Company may treat the registered owner hereof
as the owner for all purposes (notwithstanding any markings or notations on the
face of this Stock Option Grant).

      6.    Loss, Theft, Destruction, or Mutilation. Upon receipt by the
Company of satisfactory evidence of the loss, theft, destruction or mutilation
of this Stock Option Grant and either (in the case of loss, theft or
destruction) indemnification satisfactory to the Company or (in the case of
mutilation) the surrender of this Stock Option Grant for cancellation, the
Company will execute and deliver to the Investor, without charge, a Stock
Option Grant of like denomination.

      7.    Public Reporting. With a view to making available to the Investor
the benefits of Rule 144 ("Rule 144") promulgated under the Securities Act of
1933, as amended (the "1933 Act"), and any other rule or regulation of the
Securities and Exchange Commission (the "SEC") that may at any time permit the
Investor to sell Shares to the public without registration, the Company agrees
that, for so long as a class of its securities is registered under Section 12
of the Securities Exchange Act of 1934, as amended (the "1934 Act"), the
Company will: (i) make and keep public information available, as those terms
are understood and defined in Rule 144, at all times; (ii) file with the SEC in
a timely manner all reports and other documents required of the Company under
the 1933 Act and the 1934 Act; and (iii) furnish to the Investor, forthwith
upon request, (a) a written statement by the Company that it has complied with
the reporting requirements of Rule 144, the 1933 Act and the 1934 Act, (b) a
copy of the most recent annual or quarterly report of the Company filed with
the SEC and such other reports and documents so filed by the Company which the
Investor may reasonably request, and (c) such other information as may be
reasonably requested in availing the investor of any rule or regulation of the
SEC which permits the selling of any Shares without registration under the 1933
Act.

8     Notices. Any notice required or permitted to be given hereunder shall be
in writing and shall be deemed to have been sufficiently given for all purposes
if personally delivered or mailed by first class certified or registered mail,
postage prepaid, hand delivered, or sent by telecopy or by reputable overnight
courier service which requires signature upon delivery. Notices sent by

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U.S. mail shall be deemed delivered three (3) days after deposit with postal
authorities or upon confirmed delivery if personally delivered, sent by
confirmed fax or courier service. Unless otherwise specified in writing, the
mailing addresses of the Parties shall be as described below:

      For UT:     United Therapeutics Corporation
                  68 T.W. Alexander Drive
                  PO Box 14186
                  Research Triangle Park, NC 27709, USA
                  Attention: Dr. Roger Jeffs, Ph.D.
                  Fax Number: 1-919-485-8352

      For Toray:  Toray Industries, Inc.
                  2-1, Nihonbashi-Muromachi 2-chome,
                  Chou-ku, Tokyo 103-8666, JAPAN
                  Attention:  Dr. Masanobu Naruto, Ph.D.
                  General Manager of Pharmaceuticals Planning Department.
                  Fax Number:  81-3-3245-5421

      9.    Legend. The Company may cause each certificate representing Shares
issued upon exercise of this Stock Option Grant to bear legend in substantially
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."

      10.   Taxes. The Company agrees that it will pay, and will hold the
Investor harmless from any and all liability with respect to, any stamp or
similar taxes which may be determined to be payable to the State of Delaware in
connection with the issuance, delivery or exercise of this Stock Option Grant
(as well as the issuance or delivery of Shares upon any exercise of this Stock
Option Grant).

      11.   Miscellaneous. Neither this Stock Option Grant nor any term hereof
may be changed, waived, discharged or terminated orally, but only by an
instrument in writing signed by the party against which enforcement of the
change, waiver, discharge or termination is sought. The Section headings in
this Stock Option Grant are for purposes of references only and shall not limit
or otherwise affect the meaning hereof.

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      IN WITNESS WHEREOF, the Company has caused this Stock Option Grant to be
signed and delivered by its duly authorized officers as of June 27, 2000.

                              UNITED THERAPEUTICS CORPORATION,
                              a Delaware corporation

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

Witness:

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Attachment A



STOCK OPTION GRANT EXERCISE

(To be signed only upon an exercise of the Stock Option Grant)

      The undersigned, the holder of the attached Stock Option Grant, hereby
elects to exercise the purchase right represented by such Stock Option Grant
for, and to purchase thereunder, ___________ of the Shares of Common Stock of
United Therapeutics Corporation to which such Stock Option Grant relates and
herewith makes payment of $____________ therefor in cash or by certified check
or bank draft [or election is made with respect to current exercise of the
Stock Option Grant to utilize the Net Exercise Right as described in such Stock
Option Grant] and requests that the certificate for such Shares be issued in
the name of, and be delivered to ___________________________, whose address is
set forth below the signature of the undersigned.

Dated:
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                                    Signature

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                                    Title

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                                    Address

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