Treatment of guinea pig erythrocytes with types A and B influenza viruses rendered them inagglutinable by polyoma virus; also, the inhibitory effect of ovomucin on polyoma virus hemagglutination was destroyed by pretreatment of the ovomucin with various myxoviruses. These results indicate that polyoma virus and myxovirus erythrocyte receptor sites are identical. However, no destruction by polyoma virus of its own or of myxovirus receptors or inhibitors was detected. No serologic relationship was detected between polyoma virus and members of the myxovirus group; differences in size and stability further indicate their distinctness. No evidence was found of biologic or serologic relationship of polyoma virus with encephalomyocarditis virus or mouse encephalomyelitis virus.