SUMMARY: A 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 at 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% of initial infectivity for 7 days at 4° but for longer periods it was best preserved in 70 % (v/v) ethanol at 20°.


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