To determine whether tumors containing carcinoembryonic antigen could be detected by administration of a radiolabeled, affinity-purified, goat lgG having 70 per cent immunoreactivity against carcinoembryonic antigen, 18 patients with a history of cancer of diverse histopathology received an average total dose of 1.0 mCi of 131l-labeled lgG. Total-body photoscans were performed with a gamma scintillation camera at various intervals after administration of the radioactive antibody. Ordinary photoscans proved difficult to interpret because of blood-pool background radioactivity, thus necessitating the computer subtraction of radioactive blood-pool agents from the antibody's 131l activity. Tumor location could be demonstrated at 48 hours after injection in almost all cases studied. The scans were negative in patients without demonstrable tumors or with tumors apparently devoid of carcinoembryonic antigen. Circulating antigen levels of up to 350 ng per milliliter did not prevent successful tumor imaging after injection of the radioantibody.