SUMMARYA plaque assay method is described for the titration of the infective ribonucleic acid (RNA) of foot-and-mouth disease (FMD) virus. The method depends on the dilution of the RNA in salt solutions of high molarity and the treatment of the BHK 21 cell monolayers used with M-NaCl. The method is reproducible and is a t least as sensitive as other methods of titration. The extraction of RNA in the presence of EDTA gave titres corresponding to 0*1% of the titre of the intact virus. RNA prepared from virus suspensions containing EDTA retained about 50 yo of initial infectivity for 7 days at 4' but for longer periods it was best preserved in 70 % (v/v) ethanol at -20' .