Treatment of mouse L 929 cells in cell culture with mouse interferon, prior to infection with reovirus type 3, decreased the amount of virus progeny formed. In infected, interferon-treated cells less reovirus specified double-stranded RNA (ds reoRNA) and single-stranded RNA accumulated than in infected control cells and the rate of ds reoRNA accumulation was inversely related to the concentration of interferon with which the cells were treated prior to infection. Since ds reoRNA can be conveniently assayed, this forms the basis of a fast and simple assay for interferons.