The Lewis a antigen (Le a )- and Lewis b antigen (Le b )-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. Le a -5 was a major component of the purified non-acid glycolipid fraction from normal colon mucosas from four patients examined and Le a -7 was a minor component in three of these four cases. Le a in colon carcinoma of the same patients was expressed as Le a -5 in all four cases and as both Le a -5 and Le a -7 in one of the four cases. All normal colon mucosa samples expressed Le a but not Le b . Le b -6 was expressed in all tumors obtained from the same patients as normal colon mucosa (Le b negative) samples and in the gastrointestinal tumor cell lines. These findings indicate that Le b is a colon adenocarcinoma-associated antigen and that Le a and Le b are coexpressed in tumor tissue. Evidence is presented for the coexistence of Le a and Le b carbohydrate determinants in both glycolipid and glycoprotein forms in the same cells.