Expression of blood group antigens A and B was studied in a group of 48 gastric carcinomas (classified according to Laurén as intestinal and diffuse types respectively) by monoclonal antibodies in an indirect immunoperoxidase method. Results could be summarized as followings: 1. Significant difference concerned the expression of blood group antigens A and AB. Intestinal type was characterized by striking heterogeneity of expression and more pronounced deletion of antigens. Diffuse type had a homogenous expression and a distinctly smaller deletion. 2. Supranuclear immunoreactivity with monoclonal antibody HE 10 in intestinal type carcinoma may have a prognostic importance. This expression could not be observed in diffuse type carcinoma.