The Ina and Inb blood group antigens were found to be located on an erythrocyte membrane glycoprotein of 80,000 MW by immunoblotting with human anti-Ina and anti-Inb antibodies under non-reducing conditions. This glycoprotein is shown here to be identical to that defined by monoclonal antibodies to CDw44, and a new murine monoclonal antibody (BRIC 35) is added to this cluster. Experiments with endo-beta-galactosidase and Endo F preparations suggest that the glycoprotein contains one or more N-glycans but that these oligosaccharides do not contain extensive poly-N-acetyllactosaminyl sequences. Experiments using membranes prepared from sialidase-treated normal erythrocytes, from Tn erythrocytes and from Cad erythrocytes suggest that the glycoprotein does not contain a substantial content of O-glycans. The Inb antigen and the epitope defined by a murine monoclonal antibody (BRIC 35) show reduced expression on Lu(a-b-) erythrocytes which result from the effect of the dominant inhibitor gene In(Lu). Evidence is presented here that the Inb antigen is expressed on normal granulocytes and lymphocytes and on the haemopoietic cell lines HEL, K562 and HL-60, a lymphoblastoid cell line and lymphocytes from two patients with B-CLL.