Type 1 antigens (Lea and Leb) of the pyloric mucosa are restricted to the mucus cells of the surface epithelium and the neck. In the duodenum they are present in absorptive and goblet cells of duodenal villi as well as in the glands of Lieberkühn. Type 2 antigens (X, Y, and H) are also present in these areas of the mucosa, and both type 1 and type 2 antigens, in this superficial location, are under the control of the secretor gene. In addition, type 2 antigens are also present in the mucus cells of pyloric and Brunner's glands, but in this deep location they are not controlled by the secretor gene. These results suggest the existence of two differentiation patterns for the expression of glycosyltransferases in the stem cells. The upward differentiation process would favor the expression of the alpha-2-L-fucosyltransferase coded by the Se gene, and the downward differentiation process would favor the expression of the alpha-2-L-fucosyltransferase coded by the H gene. The knowledge of the distribution and the genetic control of type 1 and type 2 antigenic determinants in normal gastrointestinal tract may help to interpret the modifications in the expression of these antigens in pathologic conditions such as neoplasia.