We have previously demonstrated, using in vitro assays, a high degree of selective binding of monoclonal antibody (MAb) B72.3 for carcinomas of the colon, ovary, and breast versus normal adult tissues using in vitro assays. We report here a demonstration of selective tumor localization in colorectal cancer patients of i.v. administered 131I-labeled MAb B72.3 immunoglobulin G prior to surgery. Radiolocalization indices (RI) were obtained by direct analyses of biopsy materials (i.e., cpm of 131I-labeled MAb per g of tumor versus cpm per g of normal tissues). Using as a "positive" localization, RI of 3 times greater than normal tissue (i.e., RI greater than 3.0), tumor lesions in various sites from 17 of 20 patients scored positive. In eight of these patients, all tumor lesions demonstrated RIs of greater than 3, while in five patients RIs of some lesions were greater than 10 and as high as 30 to 46. Seventy % (99 of 142) of tumor lesions showed RIs of greater than 3, while only 12 of 210 histologically confirmed normal tissues examined showed RIs of greater than 3. These tissues, moreover, were either adjacent to tumor or draining tumor masses, or, as in the case of two patients, apparently due to high levels of circulating immune complexes that were deposited in the spleen. Positive gamma scans (confirmed at surgery) were observed in 14 of 27 patients. An isotype-identical control immunoglobulin G was coinjected and showed RIs considerably lower than that of B72.3. No toxicity or adverse reaction was observed with either MAb. These studies are among the most comprehensive to date concerning the definition of the actual delivery of radiolabeled MAb to carcinoma lesions versus a wide range of adjacent and distal normal tissues and lead the way for other diagnostic and potential therapeutic applications of this antibody either alone, or in combinations with other monoclonal antibodies.