Purified glycophorin (predominantly type A) from human erythrocytes was found to effectively inhibit reovirus hemagglutination (HA) in contrast to other glycoproteins such as fetuin or ovalbumin. Glycophorin was also a potent inhibitor of reovirus and protein sigma 1 binding to mouse L fibroblasts. Glycophorin pretreated with neuraminidase lost these inhibitory properties. Using a solid phase binding assay, it was demonstrated that reovirus as well as protein sigma 1 could specifically bind to glycophorin immobilized on polystyrene plates. This binding was inhibited by wheat germ agglutinin (WGA) but not by other lectins such as peanut agglutinin (PA), Maclura pomifera agglutinin (MPA), Bauhinia purpurea agglutinin (BPA), or concanavalin A (Con A). Binding of reovirus to glycophorin was also partially inhibited by a monoclonal antibody (10F7) (W. L. Bigbee, R. G. Langlois, M. Vanderlaan, and R. H. Jensen, 1984, J. Immunol. 133, 3149-3155), which recognizes a determinant common to the M and N forms of glycophorin, but not by N-specific monoclonal antibodies NN4 and NN5 or an M-specific monoclonal antibody, 6A7. Taken together, these results clearly indicate that the M, N blood group antigen, glycophorin, is the erythrocyte receptor for reovirus.