Monoclonal antibody B72.3 binds to a glycoprotein complex with a molecular weight of 220,000 to 400,000. B72.3 reacts with approximately 50% of human mammary carcinomas and to 80% of the colon carcinomas tested but does not react appreciably with normal mammary tissue, with normal colon, or to a variety of normal adult human tissues tested using immunohistochemical techniques. B72.3 immunoglobulin G was purified and radiolabeled with 125I without significant loss in its reactivity to tumor extracts. The radiolabeled B72.3 immunoglobulin G was shown to efficiently localize human colon carcinoma xenografts in athymic mice. Tumor:tissue ratios of the localized antibody rose over the 7-day period studied, with tumor:liver, tumor:spleen, or tumor:kidney ratios of approximately 18:1 at Day 7 and a tumor:blood ratio of approximately 5:1 at Day 7. Tumor:muscle or tumor:brain ratios rose to over 100:1. The amount of radioactivity in the tumor increased for the first 2 days postinoculation of antibody and stayed constant over a 19-day period of study. Thus, there was no appreciable loss of radioiodine from the tumor over the study interval. No localization was seen in mice bearing a B72.3 antigen-negative human melanoma xenograft or with an isotype-identical control immunoglobulin G in mice bearing colon tumor xenografts. Gamma camera imaging with a pinhole collimator confirmed the ability of the radiolabeled antibody to detect the presence of colon carcinoma xenografts less than 0.5 cm in diameter over a 19-day period. The potential use of this system as a model for radioimmunotherapy will be discussed.