Four monoclonal antibodies (MoAbs) (35, 115, 17–1A, and B723) directed towards human carcinoma surface antigens have been studied in athymic nude mice with LS174T, C0112, or SW948 colon carcinoma xenografts or negative control melanoma (MEL-1), lymphoma (Namalwa), and breast (MCF-7) carcinoma xenografts to evaluate the effects of antigenic heterogeneity and time after administration on localization and imaging. 125I-labeled 115 showed the highest uptake of any antibody in LS174T tumors. MoAbs 35 and B72.3 showed similar but lower levels of uptake in LS174T and CO112 tumors, but B72.3 concentrated less in SW948 tumors. 17-1A showed the highest degree of accumulation in SW948 tumor xenografts. No specific uptake of the four anti-carcinoma MoAbs was observed in MEL-1, Namalwa, or MCF-7 xenografts. The specificity of the in vivo tumor localization of the four anti-carcinoma MoAbs was confirmed by the low degree of accumulation of a control MoAb against influenza virus in LS174T tumors. Imaging studies with 131 I-labeled colorectal cancer MoAbs showed specific uptake and retention in LS174T tumors, with progressive clearance from the whole body. The colorectal cancer MoAbs were compared for immunohistochemical binding against biopsies from patients with colorectal cancer and adjacent normal colonic tissue. Most colorectal cancer specimens showed moderate to strong staining with the four MoAbs. The percentage of positive cells varied within and between tumors demonstrating antigenic heterogeneity. Absent to slight focal staining was seen with normal colon tissue. B72.3 showed the highest degree of staining specificity. This study indicates a difference in the immunohistochemical binding of a panel of MoAbs against biopsies of colon adenocarcinoma and a dependence of in vivo localization on the human colon cancer cell line used as target. This has important implications for future clinical diagnostic and therapeutic studies. © 1988, American Association for Cancer Research. All rights reserved.