SummaryAn erythrocyte sialoglycoprotein common antigen has been identified by a monoclonal antibody. The antigen recognised by the JCS‐2 antibody was determined by immunoprecipitation, Western blotting and partial amino acid sequencing. Western blot analysis of human erythrocyte ghost membranes separated on sodium dodecyl sulphate polyacrylamide gel electrophoresis shows that the sialoglycoproteins common antigen occurs on the alpha, beta, gamma and delta sialoglycoproteins, either in their monomeric or dimeric form and as the heterodimer in the case of alpha and delta. The N‐terminal sequence of the antigen prepared by immunoaffinity chromatography of the Triton‐X100 extract of human erythrocyte ghost shows the first 9 residues to be identical to the known sequence of the MN blood group glycoproteins. The common epitope on the sialoglycoproteins resides on the oligosaccharide and contains sialic acid.