High-titer antisera specific for the Lewis-a, -b, and -d determinants were obtained by the immunization of rabbits with artificial antigens prepared by coupling chemically synthesized haptens to bovine serum albumin. The use of the appropriate synthetic immunoadsorbents allowed the isolation of the desired antibodies from the sera and the elimination of cross-reactive antibody populations. The resulting refined antibodies proved highly effective for the localization by immunofluorescence staining of Lewis antigens in epithelial cells of gastric and duodenal tissue of H patients. Furthermore, the immunohistochemical procedures could be performed in conventionally prepared paraffin tissue sections. In erythrocyte Lewis-a and -b individuals, the strongest staining reactions were with the appropriate reagent. Lewis-a individuals were Lewis-d negative. Tissue from Lewis-a--b--secretors showed strong positivity in the epithelial cells of the stomach only with the Lewis-d reagent while none of the reagents showed strong reactions with the tissue from the one nonsecretor Lewis-a--b--patient. The significance of these results to the biogenesis of the Lewis antigens is discussed.