&NA;Large quantities of carcinoembryonic antigen (CEA) were detected by the radioimmunoassay method and confirmed by the column chromatography and immunodiffusion methods in the bowel contents from nine subjects: two healthy volunteers, two patients with colostomies and one with ileostomy free of disease, three patients with colostomies who had metastases outside the gastrointestinal tract, and one patient with carcinoma of the rectum (in place) before resection. The quantities of CEA in the intestinal contents did not correlate with the plasma levels. This may indicate that CEA could be present in large amounts in the normal epithelial cells of the gastrointestinal tract and does not appear conspicuously in blood until abnormal conditions develop.