Poliovirus radioactively labeled in its RNA and protein components was treated with various combinations of temperature, pH and sodium dodecyl sulphate (SDS) or with concentrated urea in order to induce the formation of fibrils. These were purified and their labeling was studied. In all cases the fibrils contained RNA and protein precursors. The composition calculated from the amounts of radioactivity varied widely (from 1 to 24 per cent of RNA).