Centrifugation in Percoll isopycnic gradients is reported as effective and simple method for separation and purification of polioviruses. By this method, not only may one separate poliovirus from simian virus 40 (SV40), but also may separate empty virions from solid virions of poliovirus. The bouyant density in Percoll of solid virion of poliovirus, empty virion of poliovirus and SV40 are 1.084, 1.051 and 1.13, respectively. The recovery rate of virus is about 65%.