Encephalomyocarditis (EMC) and influenza viruses attach to human erythrocytes causing hemagglutination of the cells. Sialoglycoproteins, containing predominantly glycophorin A, from these cells behave as soluble virus receptors and inhibit hemagglutination by both viruses. Removal of 43% of the sialic acid from erythrocytes with neuraminidase prevented their hemagglutination by EMC virus while loss of 40% of glycophorin sialic acid destroyed its inhibitory properties against this virus. Of the sialic acid .apprx. 80% had to be removed from erythrocytes or from glycophorin to achieve the same results for influenza virus. Trypsin treatment of erythrocytes or glycophorin had little effect on hemagglutination or inhibition involving either virus, although the glycopeptides released contain up to 70% of the total sialic acid, and despite the fact that glycophorin was drastically reduced in size as shown by SDS[sodium dodecyl sulfate]-polyacrylamide gel electrophoresis. Thus, not all of the sialic acid present in erythrocyte sialoglycoprotein receptors is involved in attachment of EMC or influenza viruses and the attachment sites on erythrocytes for these viruses are not identical.