Summary The optimal temperature for development of specific degeneration in “lipo-virus”-infected cultures was 32-34°C. Partial and complete suppression was noted at 37.5°C and 38.5°C respectively. The “lipo-virus” was lethal to chick embryos if the virus was inoculated into the amniotic or allantoic sacs or onto the chorioallantoic membrane and the embryos then incubated at 33°C. Inoculation into the yolk sac or elevation of incubation temperature to 38°C nullified the lethal action of the “lipovirus” on chick embryos.