We studied the pharmacokinetic properties of two human monoclonal antibodies to colon carcinoma cells and their ability to detect tumors in nude mice bearing primary human colon carcinoma xenografts. The 16–88 and 28A32 monoclonal antibodies are immunoglobulin M class human antibodies produced by cell lines derived from peripheral blood lymphocytes from patients with colon carcinoma. The patients received an autologous tumor cell vaccine as part of an active specific immunotherapy protocol. The 125I-labeled antibodies were cleared from the circulation of non-tumor-bearing and tumor-bearing nude mice with a 6–8-h half-life. The half-life of the antibodies in tumor tissue was 48 to 72 h compared to 8 to 12 h for normal tissues. Tumor:normal tissue ratios were highest 4 to 7 days postinjection with tumor:blood ratios of 12:1 for 16–88 and 10:1 for 28A32 antibody. Experiments with a control human immunoglobulin M myeloma protein confirmed the specificity of the human monoclonal antibodies. Radioimmunoscintigraphic studies using nude mice bearing contralateral antibody-reactive and nonreactive colon tumor xenografts further confirmed that the antibodies specifically localized in tumor tissues. The antibody-reactive tumors were clearly visible by radioimmunoscintigraphy within 4 days of injection. These experiments, undertaken as a preliminary step to clinical trials, demonstrated for the first time that i.v. administered human immunoglobulin M monoclonal antibodies could be taken up by human colon tumor tissue and retained to a sufficient extent to easily permit tumor detection by external radioimmunoscintigraphy. These studies also demonstrated that the nude mouse human colon tumor xenograft model is a useful in vivo system for comparison studies of human monoclonal antibodies as part of a selection process for clinical trials and for evaluating immunoconjugates containing these antibodies for relative pharmacokinetic properties and potential diagnostic or therapeutic efficacy.