Summary: Hemagglutination (HA) with the enteroviruses, echovirus types 3, 7, 11, 12 and 19 and coxsackievirus, group B, types 1 and 5, and with reovirus type 2 was studied. Pretreatment of suitable erythrocytes with β-glucosidase, maltase or hyaluronidase did not effect the later HA of these viruses. Trypsin treatments inhibited the red blood cells' later agglutination. Pretreatment of all these viruses with β-glucosidase removed their HA property. Coxsackie-viruses group B, types 1 and 5, were similarly altered by incubations with hyaluronidase. Trypsin and maltase did not alter the HA of any of the viruses. Inhibition of HA with these viruses was readily demonstrated with a number of sugars. The patterns and quantities of sugars necessary for these inhibition reactions were distinct for each virus. The mechanism of action of these sugars has not as yet been elucidated. These experiments make it necessary to determine chemically if the capsids of enteroviruses and reoviruses are glycoproteins.