Abstract A plaque assay for foot-and-mouth disease virus, type A, was developed in primary bovine kidney cultures, and the kinetic course of virus reproduction was established. The effect of environmental variables on plating efficiency was studied. Exposure of virus to cell layers for 90 minutes at 37° or 15 minutes at 43° prior to overlaying with agar and incubating for an additional 72 hours at 37° gave maximal plaque counts. Temperatures of 26° or 29° maintained during the adsorption period did not produce maximal numbers of plaques. Plating efficiency depended upon the volume of diluent added to virus samples before exposure to cell layers. For example, identical amounts of virus applied to cultures in volumes of 0.1, 0.4, and 6.4 ml produced plaques in the approximate relationship of 60:30:10. Virus diluent should contain serum. Without serum, calcium and magnesium ions were not fully effective. Washing cultures with phosphate-buffered saline before and after exposure to virus was not required, and it was shown that plaques developed only from infections initiated before agar was applied. Frequency distribution and dilution experiments showed that plaques were initiated by single virus particles. Where mean counts of replicate platings ranged from 47 to 66, twice the standard deviation of the means ranged from 16 to 31%. In virus reproduction experiments progeny appeared at 2.5 hours. At 11.5 hours there were 10 8.6 plaque-forming units (PFU) per millitier corresponding to a release from each cell of approximately 370 PFU.