Abstract: This invention relates to a novel method for the delivery of cytotoxic drugs to tumor cells by the administration of a tumor-specific antibody-enzyme conjugate that binds to the tumor cells, and the additional administration of a prodrug that is converted at the tumor site, in the presence of the antibody-bound enzyme, to an active cytotoxic drug. According to preferred embodiments of this invention, antibody-enzyme conjugates containing the enzyme, alkaline phosphatase ("AP"), have been used in conjunction with the novel prodrug, etoposide-4'-phosphate or 7-(2'-aminoethyl phosphate)mitomycin or a combination thereof, to effect killing of tumor cells. According to another embodiment of the invention, an antibody-enzyme conjugate containing the enzyme, penicillin V amidase ("PVA"), has been used in conjunction with a novel prodrug, N-(p-hydroxyphenoxyacetyl)adriamycin to effect killing of tumor cells.