Using monoclonal antibodies the expression of blood group antigens A and B was studied by the method of indirect immunoperoxidase reaction in a series of 48 gastric carcinomas classified by Laurén's criteria into diffuse and intestinal types. Expression in intestinal and diffuse types of carcinoma differed significantly in patients of blood group A and AB. Intestinal type was characterized by a marked heterogeneity of expression of blood group antigens, while diffuse type displayed a homogeneous expression. The observed differences testify the validity of Laurén's classification. A more frequent staining of supranuclear regions of the cytoplasm of tumor cells in intestinal type carcinoma was recorded with the monoclonal antibody HE 10 (reacting with antigenic determinants A and H of type 3 and 4). A possible prognostic significance of this finding is discussed.