One important issue in radioimmunodetection is how well the current methods can locate and disclose small metastatic foci in visceral sites. We have developed a human colonic tumor metastasis model by surgically implanting GW-39 tumor cells in the liver of unconditioned hamsters. Tumors were produced in 71 of 73 animals and were macroscopically apparent within 1 wk. In addition, multiple nodular lung metastases of GW-39 were found in about 80% of the animals given implants of tumor in the liver, but implantation of tumor in the spleen failed to show lung metastases even after 4 wk. Hamsters bearing GW-39 liver and cheek pouch grafts or normal hamsters were given injections of a mixture of 131I-labeled anti-carcinoembryonic antigen antibody and 125I-labeled irrelevant immunoglobulin G. After 7 days, tumor was localized by external scintigraphy without subtraction techniques in both the liver and cheek pouch, but even in animals with extensive lung metastases we failed to unequivocally detect tumor in the lungs by external imaging or by comparing tissue counting data from uninvolved and tumor-bearing lungs. However, whole-body autoradiography confirmed specific localization of anti-carcinoembryonic antigen antibody in the tumors at all sites indicating that tissue counting and external imaging were not sensitive enough to reveal micrometastatic tumors. Thus, the current methods used for this model appear to be useful for further investigation of the radioimaging of tumors growing in visceral organs.