EXHIBIT 5.1

                                              August 3, 2001

StemCells, Inc.
3155 Porter Drive
Palo Alto, CA 94304

Ladies and Gentlemen:

    This opinion is furnished to you in connection with a registration statement
on Form S-8 (the "Registration Statement"), filed with the Securities and
Exchange Commission under the Securities Act of 1933, as amended, on or about
the date hereof for the registration of 3,000,000 shares of Common Stock, $.01
par value (the "Shares"), of StemCells, Inc., a Delaware corporation (the
"Company"). The Shares are issuable under the Company's 2001 Equity Incentive
Plan (the "Plan").

    We are familiar with the actions taken by the Company in connection with the
Plan. For purposes of our opinion, we have examined and relied upon such
documents, records, certificates and other instruments as we have deemed
necessary.

    Based on the foregoing, we are of the opinion that, when the Shares have
been issued and sold and consideration received therefor by the Company in
accordance with the terms of the Plan, the Shares will be validly issued, fully
paid and non-assessable.

    We hereby consent to your filing of this opinion as an exhibit to the
Registration Statement.

    It is understood that this opinion is to be used only in connection with the
offer and sale of Shares while the Registration Statement is in effect.

                                          Very truly yours,

                                          /s/ Ropes & Gray

                                          Ropes & Gray