The ability of radiolabeled monoclonal antibodies to accumulate in and image small human tumors growing in the spleen of athymic mice was assessed. The antibodies B6.2 and B72.3, which reacted against human breast (Clouser) and colon (LS174T) tumor cells in vitro and in vivo, respectively, and the isotype matched anti-horseradish peroxidase antibody which did not bind to these tumors were used in pharmacokinetic and imaging experiments. Human melanoma cells and tumors (A375) which did not react with any of the three antibodies were used as additional controls. Radioiodinated "tumor specific" and non-specific antibodies were injected i.v. into athymic mice bearing intrasplenic tumors and the mice were sacrificed at various times to assess the specificity of uptake of these antibodies into tumor and normal host tissues. The accumulation of B6.2 in the Clouser tumor was maximal at 24 h as indicated by a localization index (specific/nonspecific antibody in tumor divided by the same ratio in blood) of about 4.0. The uptake of B72.3 in LS174T tumor increased with time with a localization index of about 12.0 observed at 50 h post-antibody injection. Localization indices for the control A375 tumor and for all normal mouse tissues, including the uninvolved portion of the tumor bearing spleen, were between 0.8 and 1.0, thus indicating no specific antibody accumulation. The relative blood flows of the Clouser and A375 tumors, as determined by the 86RbCl method, were similar. The results suggested that immunospecificity was a major factor in antibody localization in vivo. Specific images of approximately 100-mg Clouser tumors with radiolabeled B6.2 and of LS174T tumor with radiolabeled B72.3 were seen by 24 h after antibody injection. Images of smaller (about 20 mg) LS174T tumors were seen by 48 h following B72.3 injection. The control antibody, anti-horseradish peroxidase, did not image either Clouser or LS174T tumor. Also the control tumor was not imaged with any of the three antibodies tested. The data generated with this novel animal model support the concept of using radiolabeled monoclonal antibodies for detecting and possibly treating small metastatic visceral tumors in cancer patients.