Abstract
daltons and, b. A smaller class comprising 9.1–19.7 per cent of total RNA with the size approximately that of ribosomal 4S RNA. All IBV RNA's were fully susceptible to ribonuclease and had a buoyant density in caesium sulphate identical to that of tobacco mosaic virus RNA. No difference in the RNA profile for IBV was observed from the use of different methods of virus purification. The single-stranded RNA's of poliovirus and tobacco mosaic virus remained undegraded after preparation in the presence of IBV.