EXHIBIT 5.1


                                                        October 26, 2000


eSpeed, Inc.
One World Trade Center
103rd Floor
New York, New York 10048

                  Re:  Registration Statement on Form S-8
                       ----------------------------------

Ladies and Gentlemen:

                  We have acted as counsel to eSpeed, Inc., a Delaware
corporation (the "Company"), in connection with the preparation and filing with
the Securities and Exchange Commission (the "Commission") of a Registration
Statement on Form S-8, including the exhibits thereto (the "Registration
Statement"), under the Securities Act of 1933, as amended (the "Act"), for the
registration by the Company of an aggregate of 1,500,000 shares (the "Shares")
of Class A common stock, par value $.01 per share, of which 1,000,000 shares may
be issued under the eSpeed, Inc. Non-Qualified Employee Stock Purchase Plan (the
"Stock Purchase Plan") and 500,000 shares may be issued under the eSpeed, Inc.
Deferral Plan for Employees of Cantor Fitzgerald, L.P. and its Affiliates (the
"Deferral Plan" and together with the Stock Purchase Plan, the "Plans"), and an
indeterminate amount of interests in the Deferral Plan to be offered or sold to
participants in the Deferral Plan.

                  In connection with this opinion, we have examined originals,
or copies certified or otherwise identified to our satisfaction, of the
Registration Statement, the Plans, the Amended and Restated Certificate of
Incorporation of the Company, the Amended and Restated By-Laws of the Company,
and such other documents, records and other instruments of the Company as in our
judgment are necessary or appropriate for purposes of this opinion.

                  Based upon the foregoing, we are of the following opinion:

                  1.       The Shares, when issued by the Company in accordance
                           with the terms of the Plans and for a consideration
                           per Share of not less than the par value per Share,
                           will be duly authorized, validly issued, fully paid
                           and non-assessable.

                  2.       The Deferral Plan confers valid interests upon
                           eligible employees participating therein to the
                           extent and upon the terms and conditions described
                           therein.

                  We are hereby expressing the opinions above as members of the
Bar of the State of New York and express no opinion as to any law other than the
General Corporation Law of the State of Delaware (the "DGCL"), the applicable
provisions of the Delaware Constitution and the reported decisions interpreting
the DGCL.



                  We hereby consent to the use of this opinion as an exhibit to
the Registration Statement. In giving this consent, we do not admit that we are
acting within the category of persons whose consent is required under Section 7
of the Act or the rules and regulations of the Commission thereunder.

                                             Very truly yours,

                                             /s/ Morgan, Lewis & Bockius LLP