Summary In addition to the major infective component, which bands at a density of 1.34 g/ml in caesium chloride (‘light component’), a component with a density of 1.44 g/ml (‘heavy component’) has been found in harvests of poliovirus (type 1), Coxsackie B5 virus, a bovine enterovirus (VG-5-27) and swine vesicular disease virus (SVDV). With SVDV about 98% of the infectivity equilibrated at 1.34 g/ml but approx. 2% was present as a peak at 1.44 g/ml. The morphology of the two forms was similar but the heavy component had a smaller diameter (28 nm) than the light component (30 nm). No inter-conversion of the two forms was observed on re-cycling in fresh caesium chloride gradients and the two components had the same proportions of RNA and protein and the same polypeptide composition. Each component gave a similar proportion of the light and heavy forms on replication, but the light component had a specific infectivity about fourfold higher than that of the heavy component and was also much more efficient in eliciting the formation of neutralizing antibodies in guinea pigs. Although these results suggest that the two particles are alternative stable configurations of the virus, iodination failed to reveal any differences in the extent or pattern of labelling of the polypeptides in the two forms.