Biodistribution and radioimmunoimaging of monoclonal antibody, 5F11, against human lung adenocarcinoma (LTEP-a2) in tumor-bearing nude mice were studied. The binding rate of radioiodinated 5F11 with LTEP-a2, as determined by in vitro binding assay, was 27.64 +/- 5.01%, while the control was less than 5.0%, suggesting that 5F11 reacted specifically with LTEP-a2. Seventy-two, 96 and 120 hr after injection of 125I-5F11, the percentage of dose taken per gram tumor tissue was 4.84%, 6.29% and 6.60%, higher than those of all normal organs. From 48 to 120 hr on, the ratio of tumor to normal tissue (T/NT) in the liver, spleen, kidney, stomach and intestine was more than 2; T/NT in the lung was 3.1-4.7; T/NT in the blood increased gradually and was 1.87 at 96 after injection. Tumor location index was 4.16, while that of the normal tissue was only 1.21. These results showed that human lung adenocarcinoma xenografts in nude mice were specifically located by radioiodinated 5F11. Radioimmunoimaging was performed in tumor-bearing nude mice. The tumors were clearly visualized 72 hr after injection of 131I-5F11. Radioactivity was higher in tumor region than in other regions. The optimal imaging time was 72-96 hr after injection of radioiodinated 5F11.