The two HA moieties of reovirus (infectious particle and top component) were recentrifuged in a CsCl gradient. They were stable under these conditions and purified to such an extent that the top component HA moiety contained only 4 TCD50 per unit. Mixtures of poliovirus and reovirus (all three prototypes) were centrifuged in a CsCl density gradient. The two viruses were clearly separated; the reoviruses accumulated at a density of 1.375 g/ml. and the poliovirus at a density of 1.34 g/ml. A 100-fold relative enrichment of each virus was achieved in these density bands respectively. A similar separation of ECHO-virus 19 and reovirus 2 infectivity and hemagglutinins associated with the infectious particle was obtained, however the noninfectious (top component) hemagglutinin of these two viruses could not be separated, they both accumulated at a density of 1.30 g/ml.