The distribution of carbohydrate antigen 19-9 (CA 19-9), on pancreatic carcinomas and associated diseases correlated with carcinoembryonic antigen (CEA) localization is described. Immunohistochemical examinations were made on pancreatic specimens from 45 patients with pancreatic carcinoma. In normal pancreatic duct, the antigens were restricted to the luminal surface. In some hyperplastic epithelium, however, they were localized not only to the basolateral plasma membranes but also to the cytoplasm. In neoplastic glands, the antigens were distributed over the entire surface of the cells and in the surrounding stroma adjacent to the basal membranes of the malignant cells. The findings were compatible with previous observations of CEA and secretory component (SC) localization in gastric and colonic mucosa. In addition, the staining intensity of CA 19-9 in the routine formalin-fixed and paraffin-embedded sections was much superior to that of CEA. The relationship between CA 19-9 and other blood group antigens, such as Lewis a and Lewis b, also was discussed. Localizations of these antigens in pancreatic tissues are useful for the biologic analysis of abnormalities of the pancreatic duct epithelium, and may well facilitate pathologic diagnosis of pancreatic carcinoma.