Two rat monoclonal antibodies (BRAC 1 and BRAC 11) have been produced. BRAC 1 recognizes an epitope common to the human erythrocyte membrane glycoproteins glycophorin C (GPC) and glycophorin D (GPD). BRAC 11 is specific for GPC. Fab fragments of these antibodies and BRIC 10, a murine monoclonal anti-GPC, were radioiodinated and used in quantitative binding assays to measure the number of GPC and GPD molecules on normal erythrocytes. Fab fragments of BRAC 11 and BRIC 10 gave values of 143,000 molecules GPC per red blood cell (RBC). Fab fragments of BRAC 1 gave 225,000 molecules of GPC and GPD per RBC. These results indicate that GPC and GPD together are sufficiently abundant to provide membrane attachment sites for all of the protein 4.1 in normal RBCs.