A monoclonal antibody (iB5, IgG1 kappa) reacting with human red cells was produced after immunization of BALB/C mice with cord red cells, followed by fusion of the spleen cells with the murine myeloma cell line Ag 8-653. The monoclonal antibody agglutinated blood group N+ much better than M + N-red cells but did not recognize erythrocytes from rare individuals typed as M + N-S-s-U- and those from an En(a-) individual (M.E.P.). However, S-s-U- donors typed as M + N + or M-N+ and En(a-) red cells from donor G.W. were agglutinated. The erythrocyte receptors for iB5 are completely destroyed by papain treatment and significantly decreased by neuraminidase. Interestingly also, the iB5 antibody failed to agglutinate trypsin-treated N+M-S-s-U- erythrocytes. Other investigations have shown that the monoclonal antibody precipitated glycophorin A and B from N+ red cells and only glycophorin B from M+N-erythrocytes. The reactivity of iB5 was further explored by immunostaining following the electrophoretic transfer to nitrocellulose sheets of membrane proteins from common (M and N) and rare erythrocytes [En(a-),S-s-U-, MgMg, McM, St(a+), Mi.V, Mi.III, Tn] separated by SDS-polyacrylamide gel electrophoresis. These studies have clearly demonstrated that the monoclonal iB5 antibody is directed against the homologous N-terminal domain of glycophorin A and B, a specificity which explains the serological reactivity of iB5 against common and rare erythrocytes.