The chemical nature of BK virus receptors on human eryrthrocytes was investigated. Glycosidases, KIO4, proteases, phospholipases and other substances acting specifically on different structures of plasma membranes were used to ascertain the role of surface cell components in the interaction with BK virus. An analogy between BK virus receptors on red cells and non-antibody serum inhibitors active against hemagglutination by the same virus is indicated.