CEA activity was studied using a three-layer bridge immunoperoxidase method in paraffin-embedded histological sections of gastric mucosa from patients with gastric cancer and from normal asymptomatic subjects. In all cases gastric cancer cells showed CEA or CEA-like activity irrespective of the histological type of cancer. The activity was located mainly in cellular membranes, on the brush border or in apical parts of the cell cytoplasm. CEA activity could not be demonstrated in mucosa specimens from normal controls. These results indicate that the appearance of CEA or CEA-like substances is regularly associated with the malignant transformation of epithelial cells of the gastric mucosa.