The LS-174T human colon carcinoma line and A375 human melanoma line were used to establish primary tumor xenografts at three sites (subcutaneous, spleen, and kidney) in congenitally athymic mice. A monoclonal antibody (MAb) reactive with the LS-174T line, B72.3 IgG, was labeled with iodine 125, and an isotype-identical control antibody MOPC-21, was labeled with iodine 131. Labeled antibodies were injected intravenously in tumor-bearing mice, and animals were killed at varying intervals. Tumor-to-blood and tumor-to-organ ratios of MAb 72.3 indicated no significant difference at any of the three primary tumor sites in LS-174T tumor-bearing mice. The percent injected dose per gram was higher at visceral sites on day 3, but was similar on days 5 and 7 at all sites. Localization indices on all days ranged from 4 to 1 to greater than 16 to 1, confirming the specificity of the B72.3 reactivity at all sites. Athymic mice bearing tumor xenografts were scanned on day 7, and the LS-174T spleen and kidney tumors were imaged, with efficacy similar to that of the subcutaneous site. The visceral tumor model is more representative of the human disease, and may therefore be a better model for evaluation of monoclonal antibodies for radioimmunodetection and therapy for cancer in intra-abdominal organs.