A phase I/II prospective clinical trial was performed with indium-111-labeled monoclonal antibody (MoAb) conjugate B72.3-glycyl-tyrosyl N-epsilon-diethylenetriaminepentaacetic acid (CYT-103) in 28 preoperative patients with biopsy-proved or suspected colorectal carcinomas. Immunoscintigraphy was performed 2-7 days after infusion of 4.1 mCi (152 MBq) of In-111 labeled to CYT-103 at doses of 0.5, 1.0, 2.0, and 20.0 mg. Surgical and histologic confirmation was available in all cases. Use of In-111 CYT-103 made possible detection of 75% of colorectal carcinomas at doses of 1.0 mg and higher, compared with only 20% detection at the 0.5-mg MoAb dose. Immunohistochemical staining for tumor-associated glycoprotein (TAG)-72 in resected carcinoma tissues demonstrated a positive correlation between MoAb imaging and the percentage of cells that expressed TAG-72. One patient suffered an adverse reaction after MoAb infusion. Human antimouse response to CYT-103 developed in 16% of patients.