THE MEDICINES COMPANY 2000 EMPLOYEE STOCK PURCHASE PLAN

Exhibit 10.1

THE MEDICINES COMPANY

2000 EMPLOYEE STOCK PURCHASE PLAN ¹

     The following constitute the provisions of the 2000 Employee Stock Purchase Plan of The Medicines Company.

     1. PURPOSE. The purpose of the Plan is to provide employees of the Company and its Designated Subsidiaries with an opportunity to purchase Common Stock of the Company through accumulated payroll deductions. It is the intention of the Company to have the Plan qualify as an “Employee Stock Purchase Plan” under Section 423 of the Internal Revenue Code of 1986, as amended. The provisions of the Plan, accordingly, shall be construed so as to extend and limit participation in a manner consistent with the requirements of that section of the Code.

     2. DEFINITIONS.

          a. “BOARD” shall mean the Board of Directors of the Company.

          b. “CODE” shall mean the Internal Revenue Code of 1986, as amended.

          c. “COMMON STOCK” shall mean the Common Stock of the Company.

          d. “COMPANY” shall mean The Medicines Company and any Designated Subsidiary of the Company.

          e. “COMPENSATION” means the amount of money reportable on an Employee’s Federal Income Tax Withholding Statement (Form W-2) before any withholdings for health insurance or under a Section 401(k), 125, 129 or similar plan, including without limitation, salary, wages, overtime, shift differentials, bonuses and incentive compensation, but excluding third party sick or disability pay, allowances and reimbursements for expenses such as relocation allowances or travel expenses, whether specifically designated as such or designated as signing bonuses, income or gains attributable to restricted stock, stock options, stock appreciation rights or other similar equity based compensation, imputed income for non cash items, such as life insurance premiums, and similar items, whether or not specifically itemized on the Form W-2.

          f. “DESIGNATED SUBSIDIARY” shall mean any Subsidiary which has been designated by the Board from time to time in its sole discretion as eligible to participate in the Plan.

          g. “EMPLOYEE” shall mean any individual who is an employee of the Company for tax purposes whose customary employment with the Company is more than five (5) months in any calendar year. For purposes of the Plan, the employment relationship shall be treated as continuing intact while the individual is on sick leave or other bona fide leave of absence approved by the Company. If a sick leave or other leave of absence exceeds 90 days and the individual’s right to reemployment is not guaranteed either by statute or by contract, the individual’s employment relationship with the Company shall be deemed to have terminated on the 91st day of such leave.

          h. “ENROLLMENT DATE” shall mean the first day of each Offering Period.

          i. “EXERCISE DATE” shall mean the last Trading Day of each Offering Period.

          j. “FAIR MARKET VALUE” shall mean the value of the Common Stock on any given date of determination, determined as follows:

               (1) If the Common Stock is listed on any established stock exchange or a national market system, including without limitation The Nasdaq National Market or The Nasdaq Small Cap Market of The Nasdaq Stock Market, its Fair Market Value shall be the closing sales price for such stock (or the closing bid, if no sales were reported) as quoted on such exchange or system on the date of such determination, as reported in The Wall Street Journal or such other source as the Board deems reliable; or

               (2) If the Common Stock is regularly quoted on the over-the- counter market, its Fair Market Value shall be the mean of the closing bid and asked price for such stock as quoted on such market on the date of such determination, as reported in The Wall Street Journal or such other source as the Board deems reliable; or

               (3) In the absence of an established market for the Common Stock, the Fair Market Value of the Common Stock shall be determined in good faith by the Board.

          k. “OFFERING PERIODS” shall mean the periods of approximately six (6) months during which payroll deductions will be made and held for the purchase of Common Stock at the end of the Offering Period, commencing on the first Trading Day on or after September 1 and March 1 of each year and terminating on the last Trading Day in the periods ending six (6) months later; provided, however, that the first Offering Period under the Plan shall commence with the first Trading Day of the month first commencing after the date on which the Common Stock is first traded on the Nasdaq National Market (such date referred to as the “First Offering Commencement Date”); and provided further that if the number of days between the date the Common Stock is first traded and the First Offering Commencement Date is less than 15 days, then the first Offering Period shall commence on the first Trading Day of the following month. The duration and timing of Offering Periods may be changed pursuant to Section 4 of this Plan.

          l. “PLAN” shall mean this Employee Stock Purchase Plan.

          m. “PURCHASE PRICE” shall mean eighty-five percent (85%) of the Fair Market Value of a share of Common Stock on the Enrollment Date on an Offering Period or on the Exercise Date for such Offering Period, whichever is lower.

          n. “RESERVES” shall mean the number of shares of Common Stock covered by each outstanding option under the Plan which has not yet been exercised and the number of shares of Common Stock which have been authorized for issuance under the Plan but not yet placed under option.

          o. “SUBSIDIARY” shall mean a corporation, partnership, limited liability company or similar entity, whether domestic or foreign, of which not less than 50% of the voting interests are held by the Company or a Subsidiary, whether or not such entity now exists or is hereafter organized or acquired by the Company or a Subsidiary.

          p. “TRADING DAY” shall mean a day on which the established stock exchange, the national market system or the over-the-counter market on which the Common Stock is traded is open for trading.

     3. ELIGIBILITY.

          a. Any Employee who shall be employed by the Company at least seven (7) calendar days prior to a given Enrollment Date shall be eligible to participate in the Offering Period commencing on such Enrollment Date.

          b. Any provisions of the Plan to the contrary notwithstanding, no Employee shall be granted an option under the Plan (i) to the extent that, immediately after the grant, such Employee (or any other person whose stock would be attributed to such Employee pursuant to Section 424(d) of the Code) would own capital stock of the Company and/or hold outstanding options to purchase such stock possessing five percent (5%) or more of the total combined voting power or value of all classes of the capital stock of the Company or of any Subsidiary, or (ii) to the extent that his or her rights to purchase stock under all employee stock purchase plans of the Company and its Subsidiaries accrues at a rate which exceeds Twenty-Five Thousand Dollars ($25,000) worth of stock (determined at the Fair Market Value of a share of Common Stock at the time such option is granted) for each calendar year in which such option is outstanding at any time. In the event that an Employee may not be granted an option under the Plan because of the foregoing restrictions, the Employee shall be granted an option to purchase the maximum number of shares that would not violate the foregoing restrictions.

     4. OFFERING PERIODS AND PURCHASE PERIODS. The Board shall have the power to change the duration of Offering Periods (including the commencement dates thereof) with respect to future offerings without stockholder approval if such change is announced at least five (5) days prior to the scheduled beginning of the Offering Period to be affected thereafter.

     5. PARTICIPATION.

          a. An eligible Employee may become a participant in the Plan by completing a subscription agreement in the form as the Company may deem satisfactory and filing it with the Company’s payroll office or such other office as the Company may direct prior to the Enrollment Date for the applicable Offering Period.

          b. Payroll deductions for a participant shall commence on the first payroll following the Enrollment Date for the applicable Offering Period and shall end on the last payroll in such Offering Period, unless sooner terminated by the participant as provided in Section 9 hereof.

     6. PAYROLL DEDUCTIONS.

          a. At the time a participant files his or her subscription agreement, he or she shall elect to have payroll deductions made on each payday during the Offering Period in an amount not exceeding ten percent (10%) of the Compensation which he or she receives on each payday during the Offering Period.

          b. All payroll deductions made for a participant shall be credited to his or her account under the Plan and shall be withheld in whole percentages only. A participant may not make any additional payments into such account.

          c. A participant may discontinue his or her participation in the Plan as provided in Section 9 hereof, or may increase or decrease the rate of his or her payroll deductions to not more than ten percent (10%) or less than zero percent (0%) not more than one (1) time during each Offering Period by completing or filing with the Company a new subscription agreement authorizing such change in payroll deduction rate. The Board may, in its discretion, increase or decrease the number of participation rate changes that may be made by a participant during any Offering Period. The change in rate shall be effective with the first full payroll period following the fifth (5th) business day after the Company’s

receipt of the new subscription agreement. A participant’s subscription agreement shall remain in effect for successive Offering Periods unless terminated as provided in Section 9 hereof.

          d. At the time the option is exercised, in whole or in part, or at the time any of the Company’s Common Stock issued under the Plan is disposed of, the participant must make adequate provision for the Company’s federal, state, or other tax withholding obligations, if any, which arise upon the exercise of the option or the disposition of the Common Stock. At any time, the Company may, but shall not be ob