Cal. Health & Safety Code § 7150.45

(a) A person authorized to make an anatomical gift under Section 7150.40 may make an anatomical gift by a document of gift signed by the person making the gift or by that person’s oral communication that is electronically recorded or is contemporaneously reduced to a record and signed by the individual receiving the oral communication.
(b) Subject to subdivision (c), an anatomical gift by a person authorized under Section 7150.40 may be amended or revoked orally or in a record by any member of a prior class who is reasonably available. If more than one member of the prior class is reasonably available, the gift made by a person authorized under Section 7150.40 may be amended or revoked as follows:
1. (1) Amended only if a majority of the reasonably available members agree to the amending of the gift.
2. (2) Revoked only if a majority of the reasonably available members agree to the revoking of the gift or if they are equally divided as to whether to revoke the gift.
(c) A revocation under subdivision (b) is effective only if, before an incision has been made to remove a part from the donor’s body or before invasive procedures have begun to prepare the recipient, the procurement organization, transplant hospital, or physician or technician knows of the revocation.