The buoyant density of dense poliovirus particles has been examined in density gradients other than cesium chloride in order to determine the dependence of this property on the nature of the solvent. In Urografin (sodium and methylglucamine amidotrizoate), dense poliovirus particles banded at two densities--1.33 and 1.39 g/cm(3)--whereas in cesium metrizoate they banded only at 1.39 g/cm(3) and in cesium sulfate at 1.38 g/cm(3). The presence of cesium ions gives rise to the occurrence of dense particles, which are indistinguishable in their buoyant density and sedimentation coefficient from standard poliovirus particles when prepared in the absence of Cs+. In their physical properties dense poliovirus particles are more closely related to rhinoviruses and foot-and-mouth-disease virus, i.e., to members of the genus Rhinovirus, than to standard poliovirus particles belonging to the genus Enterovirus.