Monoclonal antibody (moAb) B72.3 appears to be exceptional among the many mouse moAbs reactive with human tumor cells in its degree of tumor specificity. The antigen recognized was characterized as a mucin-like molecule. We report here that B72.3 reacts strongly with ovine submaxillary mucin and that reactivity is abolished by desialylation, suggesting that the determinant recognized is the disaccharide N-acetylneuraminic acid alpha (2----6)-N-acetylgalactosamine, linked to serine or threonine, which might be designated sialylated Tn. A variety of human mucin preparations, including human salivary mucins, and other glycoproteins containing O-linked or N-linked carbohydrates were nonreactive. The data suggest that the determinant recognized is a core structure which is normally not exposed due to chain elongation at C-3 of the N-acetylgalactosamine, which appears to be less frequent in carcinoma cells than in normal cells.