Upon treatment with nitrous acid, southern bean mosaic virus (SBMV) was inactivated twice as fast as its isolated RNA. Nitrous acid treatment of SBMV resulted in appreciable binding of the virus protein to the nucleic acid. The extent of deamination of aminoribonucleotides, of intraparticle RNA and isolated SBMV-RNA by nitrous acid was identical. SBMV and SBMV-RNA were inactivated at the same rate following exposure to u.v. light. Furthermore, the extent of photoreactivation of the u.v.-irradiated SBMV and SBMV-RNA was similar. U.v. irradiation did not induce any detectable binding of virus protein to RNA in SBMV.