A quantitative colorimetric method for measuring the inhibition of viral cytopathic effects has been adapted to the assay of antiviral compounds. Drug-treated, virus-infected cultures in microtiter plates were stained with the vital dye neutral red and the amount of dye incorporated was determined in a multichannel spectrophotometer. The technique required smaller volumes of reagents, was more easily automated than the standard plaque reduction assay and had good reproducibility. Standard conditions of 30 infectious units of challenge virus and 72-h incubation were judged to be optimal. Median inhibitory concentrations (ID50) for a number of compounds were approximately tenfold higher in the dye-uptake assay compared with the plaque reduction assay, possibly related to the higher multiplicity of infection required to give the desired level of cytopathic effect in the microtiter method.