A lectin, Crenomytilus grayanus (CGL), was purified from sea mussel C. grayanus by affinity chromatography on acid-treated Sepharose 6B and following gel filtration on Sephacryl S-200. Molecular weight of the lectin obtained was determined by SDS-PAGE to be 18,000, independent of the presence or absence of beta-mercaptoethanol. CGL was found to agglutinate all types of the human erythrocytes together with mouse and rabbit. In hemagglutination inhibition assays, N-acetyl-D-galactosamine, D-galactose, and D-talose were the most potent inhibitors among the monosaccharides tested. Out of the oligosaccharides containing nonreducing terminal D-galactose, melibiose, and raffinose were found to be strong inhibitors. On the other hand, among the glycoproteins, asialo-BSM was the best inhibitor. The hemagglutinating activity of CGL was independent of the divalent cations Ca2+ and Mg2+. Significant CGL activity was observed between pH 8-10.