Six different hybridoma cell lines were obtained producing IgM monoclonal antibodies (MAbs) against human blood group A antigen. All these MAbs agglutinate the A1, A1B and A2 erythrocytes, while only one of them agglutinates the A2B erythrocytes. The agglutination activity of 3 of these MAbs is inhibited by N-acetyl-D-galactosamine (D-GalNAc). Dissociation constants of complexes of these MAbs with D-GalNAc are approximately 2.2 X 10(-3) mol/l.