Summary Both fowl plague and Newcastle disease viruses failed to produce interferon in chick-embryo cells because of the inhibitory effect of infection on host-cell protein synthesis. When this inhibitory effect was destroyed by ultraviolet irradiation or by hydroxylamine inactivation, interferon was formed. No virus RNA synthesis could be detected in the interferon-producing cells and it was concluded that the virus single-stranded RNA was the stimulus for interferon formation.