Purified poliovirus preparations were heated and analysed by sucrose gradient centrifugation. They consisted of virus-like particles containing RNA and sedimenting at about 80s, empty 80s capsids, 35s virus RNA, and a non-sedimentable capsid polypeptide (VP 4). By electron microscopy the 80s ribonucleoprotein particles (80s RNP) were similar in appearance to intact poliovirus particles. They contained infectious, RNase-sensitive RNA that could be liberated from the capsid by treatment at room temperature with 1% sodium dodecyl sulphate. One of the virus polypeptides was missing, and they had lost the antigenicity of the mature virus and the ability to adsorb to HeLa cells.