The erythrocyte receptor on rhesus macaque erythrocytes used by canine parvovirus (CPV) for binding in hemagglutination (HA) was examined. Erythrocyte membrane proteins were electrophoresed and blotted to nitrocellulose and probed with [125I]-labeled CPV capsids, showing seven virus-binding proteins. Treatment of erythrocytes or isolated membranes with Clostridium perfringens neuraminidase virtually abolished virus binding. Binding was also affected by treatment with potassium periodate and inhibited by wheat germ agglutinin, but was not significantly affected by concanavalin A, peanut agglutinin, or soluble N-acetyl-neuraminlactose. A non-HA mutant of CPV failed to bind to erythrocytes or to blotted erythrocyte membrane proteins. The mutation was a single Arg-Lys difference of VP2 amino acid residue 377. The pH dependence of binding of the closely related feline panleukopenia virus was shown to result from a decreased binding in buffers with pH values of 6.8 or greater. The VP2 residues responsible for that difference have been shown to be 323 and 375. The sequences affecting binding were all adjacent to the dimple in the capsid, implicating that region of the capsid as the sialic acid binding site. The role of sialic acid in virus-host cell interactions was not defined, but the plaque sizes of the non-HA mutant and wild type CPV were indistinguishable.