Abstract Brome mosaic virus degraded to RNA in 0.02 M tris buffer, 0.001 M EDTA, pH 7–9, containing bentonite and 0.1 M NaCl. The virus coat protein was apparently adsorbed by the bentonite. In bentonite in buffer without NaCl, brome mosaic virus degraded to three nucleoproteins, each containing one piece of RNA. The largest nucleoprotein was infective. Tobacco mosaic virus degraded to RNA and protein in the above buffer with bentonite, but degraded only to short, variable length rods when 0.1 M NaCl was added. Tobacco mosaic virus coat protein apparently was not adsorbed by the bentonite. Tobacco mosaic virus RNA prepared by bentonite treatment and density-gradient centrifugation had about 20% the specific infectivity of RNA in intact virions.