PROSPECTUS SUPPLEMENT                                         File No. 333-68747
(To Prospectus and Prospectus Supplement dated May 6, 1999)       Rule 424(b)(3)
Prospectus number: 1959



                            Merrill Lynch & Co., Inc.
                           Medium-Term Notes, Series B
                   Due Nine Months or More from Date of Issue


                               Floating Rate Notes


Principal Amount:   $5,000,000         Original Issue Date:     April 28, 2000

CUSIP Number:       59018S 6KS         Stated Maturity Date:    April 18, 2005

Interest Calculation:                  Day Count Convention:
- ---------------------                  ---------------------
|x|  Regular Floating Rate Note        |x|  Actual/360
| |  Inverse Floating Rate Note        | |  30/360
     (Fixed Interest Rate):            | |  Actual/Actual

Interest Rate Basis:
- --------------------
|x|  LIBOR                             | |  Commercial Paper Rate
| |  CMT Rate                          | |  Eleventh District Cost of Funds Rate
| |  Prime Rate                        | |  CD Rate
| |  Federal Funds Rate                | |  Other (see attached)
| |  Treasury Rate
    Designated CMT Page:                   Designated LIBOR Page:
      CMT Telerate Page:                     LIBOR Telerate Page:          3750
      CMT Reuters Page:                      LIBOR Reuters Page:


Index Maturity:          Three Months    Minimum Interest Rate:   Not Applicable

Spread:                  0.1250%         Maximum Interest Rate:   Not Applicable

Initial Interest Rate:   TBD             Spread Multiplier:       Not Applicable



Interest Reset Dates:      Quarterly,  on the 18th of January,  April,  July and
                           October,   commencing  July  18,  2000;   subject  to
                           following business day convention;  initial coupon is
                           interpolated between 2 & 3 month libor rate

Interest Payment Dates:    Quarterly,  on the 18th of January,  April,  July and
                           October,   commencing  July  18,  2000;   subject  to
                           following business day convention;  initial coupon is
                           interpolated between 2 & 3 month libor rate

Repayment at the
Option of the Holder:      The  Notes  cannot  be  repaid  prior  to the  Stated
                           Maturity Date.

Redemption at the
Option of the Company:     The Notes  cannot  be  redeemed  prior to the  Stated
                           Maturity Date.

Form:                      The  Notes  are  being  issued  in  fully  registered
                           book-entry form.

Trustee:                   The Chase Manhattan Bank

Dated:                     April 25, 2000