The Lewis a antigen (Lea)- and Lewis b antigen (Leb)-active glycolipids and glycoproteins in normal colon mucosa, colon carcinoma tumors, and gastrointestinal tumor cell lines were studied by using monoclonal antibodies against Lewis determinants in solid-phase radioimmunoassay, chromatogram binding assay, and immunoprecipitation/PAGE. Lea-5 was a major component of the purified non-acid glycolipid fraction from normal colon mucosas from four patients examined and Lea-7 was a minor component in three of these four cases. Lea in colon carcinoma of the same patients was expressed as Lea-5 in all four cases and as both Lea-5 and Lea-7 in one of the four cases. All normal colon mucosa samples expressed Lea but not Leb. Leb-6 was expressed in all tumors obtained from the same patients as normal colon mucosa (Leb negative) samples and in the gastrointestinal tumor cell lines. These findings indicate that Leb is a colon adenocarcinoma-associated antigen and that Lea and Leb are coexpressed in tumor tissue. Evidence is presented for the coexistence of Lea and Leb carbohydrate determinants in both glycolipid and glycoprotein forms in the same cells.