Abstract: The instant invention relates to a novel method for the delivery of antitumor drugs to tumor cells by the administration of a tumor-selective antibody-.beta.-lactamase conjugate that binds to tumor cells, and the additional administration of a cephalosporin prodrug that is converted at the tumor site, in the presence of the antibody-.beta.-lactamase, to an active cytotoxic drug. According to the preferred embodiment of this invention, a cephalosporin mustard has been constructed which when cleaved by .beta.-lactamase yields a cytotoxic nitrogen mustard. The methods, antibody-enzyme conjugate, prodrugs, pharmaceutical compositions, and combinations of this invention provide for enhanced selective killing of tumor cells and are thus useful in the treatment of cancers and other tumors.