This study reports the use of radiolabeled antibodies to alpha-fetoprotein for the detection and localization of hepatocellular and germ cell carcinomas. Twelve patients with histories of histologically-confirmed neoplasia received a total dose between 1.0 and 4.4 mCi of 131I-labeled goat IgG prepared against human alpha-fetoprotein. Total-body photoscans were taken with a gamma scintillation camera at various intervals after injection of the radioactive antibody. Computer subtraction of radioactive technetium background images from the antibody 131I scans permitted the visualization of all tumor sites known to be present in 4 patients with either primary hepatocellular cancer or metastatic germ cell carcinoma of the testis. In contrast to the results with the specific antibody, radioactive normal goat IgG given to one of these patients with metastatic embryonal carcinoma of the testis failed to show equivalent localization. Among 8 patients with diverse neoplasms not believed to contain alpha-fetoprotein, 5 of 19 tumor sites showed radioactive antibody accretion, although significantly less than in the patients with liver or testicular cancer. Circulating antigen levels of up to 15,000 ng per milliliter did not prevent successful tumor imaging after intravenous injection of the radioantibody. This investigation indicates that alpha-fetoprotein-containing tumors can be detected and localized in vivo by the method of radioimmunodetection.