Present study was undertaken to investigate accumulation of radiolabeled antibody to CEA in CEA-producing tumor and possibility of radioimmunodetection of cancer with antibody. Nine patients with gastric carcinoma and seventeen colorectal carcinoma were studied. Antibody to CEA were produced in horse by immunization with CEA purified from hepatic metastasis of colorectal carcinoma. The purified antibodies were labeled with 125I or 131I. The patients who entered these studies received radioactive antibodies to CEA intravenously 24-48 hours prior to operation. Radioactivity in tumor tissues and normal mucosal tissues obtained at the operation was counted by gamma-scintillation counter. In four patients with liver metastasis from colorectal carcinoma, external photoscanning was also performed for radioimmunodetection of cancer with 131I-labeled antibody to CEA. The results were as follows. The mean ratio of 125I-uptake of cancer tissue to that of normal mucosal tissue was 3.28 +/- 1.52 (n = 26), and ratio to peripheral blood, liver, skin and fat tissue was 1.91 +/- 1.32, 2.90 +/- 1.47, 5.70 +/- 3.28 and 12.86 +/- 8.48, respectively. Tumor tissue showed the highest uptake. Accumulation of radiolabeled antibody to CEA in tumor tissue was higher in patients with relatively high serum CEA level. High level of circulating CEA did not hinder accumulation of radiolabeled antibody to CEA in tumor tissue. In two out of four patients with liver metastasis from colorectal carcinoma, the tumor location was successfully demonstrated by external photoscanning using gamma-camera. The information obtained in the present study provided possibility of direct radioimmunodetection and immunochemotherapy.