The occurrence of carcinoembryonic antigen (CEA) was studied in 45 cases of gastric tumors by the immunoperoxidase technique. CEA-positive staining was found in 48.8% of tumors. A correlation was found between preoperative CEA values and tumor CEA staining. All patients with serum CEA values below 2.5 ng/ml showed CEA-negative staining of tumor. In patients with serum CEA values between 2.6 and 10 ng/ml, the tumors showed a minority of CEA-positive cells; but in patients with serum CEA values above 10 ng/ml, the tumors contained a majority of CEA-positive cells. CEA-positive staining was found in 34.4% of tumors of the diffuse type, and in 75% of tumors of the intestinal type. A high percentage of CEA positivity was seen in well-differentiated tumors (87.7%) compared to the moderately differentiated (69.2%), and to the undifferentiated (28.7%). A faint CEA-positivity was observed in intestinal metaplasia, while normal gastric mucosa was CEA-negative.