Purified, [131I]-labeled goat antibodies against carcinoembryonic antigen, which have been shown to localize in human carcinoma in nude mice, were injected into 27 patients with carcinoma. Patients were scanned with a scintillation camera at various intervals. In 11 patients, radioactivity was detectable in the tumor 48 hours after injection. Computerized subtraction of blood-pool radioactivity provided clearer pictures in positive cases, but in 16 patients the scans remained doubtful or negative. To study the specificity of [131I]-antibody localization, we gave some patients simultaneous injections of [125I]-labeled normal IgG. Both isotopes were measured by means of scintillation counting in tumors and normal tissues recovered after surgery. The results demonstrated that only the anti-CEA antibodies localized in tumors. However, the total antibody-derived radioactivity in the tumor was only about 0.001 of the injected dose. We conclude that, despite the present demonstration of specificity, this method of tumor detection is not yet clinically useful.