131I-labeled goat immunoglobulin G (IgG) prepared against carcinoembryonic antigen (CEA) was administered at an average dose of 1.0 mCi (180 to 250 microgram IgG protein) to patients with ovarian tumors in order to evaluate this method of tumor detection and localization, termed the radioimmunodetection of cancer. All primary cancers in 13 patients could be localized, whereas the metastases in six of nine cases could be imaged by external scintigraphy. However, only two of these cases showed metastatic spread by more conventional diagnostic techniques, including computer-assisted tomography, ultrasonography, and angiography. Successful tumor radiolocalization appeared to depend on tumor size, with lesions smaller than 2 cm in diameter not being detected. Tumors containing a CEA concentration above 115 ng/g, including a benign neoplasm, could be localized with radioactive anti-CEA antibodies. Administration of radioiodinated normal goat IgG to four patients with malignant or benign ovarian tumors failed to show tumor radioimmunodetection. One of these cases subsequently demonstrated a 4- x 4-cm tumor after receiving specific radiolabeled anti-CEA IgG. This study shows that ovarian neoplasms containing CEA can be detected and localized by external photoscanning after the application of radioiodinated antibodies to CEA and that, in this small series of patients, primary and secondary tumors could be detected in 100 and 67% of the cases, respectively.