Intravenous injections of 131-I-labeled anti-EL4 lymphoma antibodies showed progressive localization of radioactivity in EL4 transplants but not in B16 melanoma in mice carrying both tumors. Normal rabbit globulin labeled with 131-I did not localize in either tumor and cleared more slowly from the internal organs. Metastatic localization of intravenous 131-I-labeled anti-tumor antibodies was also observed in 2 cancer patients.