The semi-micro method for interferon assay was based on quantitation of inhibition of cytopathic effects in 6-mm wells. Cultures were exposed to interferon dilutions, challenged with vesicular stomatitis virus, and stained with methylrosanaline chloride. The bound dye was then eluted and measured colorimetrically. The method was more sensitive than the plaque-inhibition method and was extremely economical. Culture age was the most important factor affecting reproductibility.