Eighty-eight monoclonal antibodies were tested by immunohistochemistry on human gastro-duodenal mucosae of known ABO, Lewis and Secretor phenotypes. Antibodies were classified among anti-A, anti-B, anti-AB, anti-H and other anti-glycoconjugates (I, i, T, Tn, Lewis, P1, Tk). Anti-A, B and AB antibodies were subdivided into subgroups with "broad" or "restricted" reactivity according to the extent of epithelial cell labeling. Anti-H antibodies were classified in accordance to their degree of sensitivity to the secretor phenotype. Among anti-T and anti-I antibodies, only one of each showed positive staining of epithelial cells. All anti-Lewis antibodies had distinct reactivities, although, they were clearly anti-Lewis reagents. Some anti-P1 antibodies labeled epithelial cells, irrespective of the ABO, Lewis and secretor phenotypes. One anti-Tk stained the Golgi apparatus of most epithelial cells, irrespective of the individual's phenotype. In conclusion, some of the antibodies tested were defined as very useful reagents for immunohistochemistry showing both specificity and sensitivity.