Quantitative assay of interferon by the immunofluorescent cell-counting technique.

A rapid, quantitative assay of interferon was developed, based on the reduction of the number of virus-infected cells by dilutions of interferon. Infected cells were enumerated by fluorescent cell-counting 24 hours after challenge of interferontreated McCoy cover-slip cell monolayers. A quantitative linear relationship between the reduction of fluorescent cells and dilutions of interferon was demonstrated by probit transformation of inhibition percentages. Yellow fever virus, selected as the standard challenge virus, was the most sensitive of four viruses tested to the action of interferon prepared in McCoy cell cultures.