Dye Uptake Methods for Assessing Viral Cytopathogenicity and their Application to Interferon Assays

When neutral red is added to the liquid medium in tissue cultures, more dye is taken up by healthy cells than by damaged cells. This forms the basis of a method for assessing the relative extent of cell damage in different cultures, the amounts of bound dye being determined directly by inspection of the stained cell sheets, or indirectly by estimating extracted dye colorimetrically. This procedure can be used to measure the effects of a cytocidal virus and can also be applied to the assay of interferons. Here it provides a precise and reproducible method which is unusually sensitive for the assay of mouse interferon, and has also been used to assay rat, chick, rabbit and human interferons.