Dense poliovirus particles (buoyant density 1.44 g/ml in CsCl) isolated from infected HeLa cells contain the normal four structural polypeptides VP1 to VP4, and 35S poliovirus RNA. In addition, small amounts of VPo and single-stranded RNA sedimenting slower than the poliovirus genome are present. Dense particles have a low specific infectivity, are neutralized by type-specific poliovirus antisera, and are detected during growth as early as normal virus but disappear when virus production stops. They appear to represent a different, more open, conformation of the normal virus capsid.