In clinical studies we have evaluated a unique monoclonal antibody-based drug delivery system, a bifunctional antibody designed to deliver imaging or therapeutic agents, such as radioisotopes, drugs, or biologics, to tumor cells, while minimizing the dose to normal tissue. The bifunctional antibody, with one specificity to a tumor-associated antigen (carcinoembryonic antigen) and another specificity to a hapten, is injected and allowed to localize at a tumor site for 4 days. A hapten, tagged with a radioisotope, is subsequently injected for delivery to and capture by the prelocalized antibody at the tumor site. In studies reported here, the sulfhydryl groups of Fab' fragments of ZCE-025 and CHA-255 were linked with bis-maleimidomethyl ether to form an F(ab')2 bifunctional antibody coupled by a stable tioether linkage. EOTUBE, a hydroxyethylthiourido derivative of benzyl EDTA, was used as the hapten carrier of 111In. Fourteen patients 62–82 years old with recurrent or metastatic adenocarcinoma of the colon were studied. Twenty of 21 known lesions were imaged, and eight of nine new lesions were confirmed. With this fundamentally new approach to drug delivery, clearance from normal tissue is rapid, and high tumor:normal tissue ratios are expeditiously achieved.