Abstract
Actinomycin D was found to inhibit interferon formation in rabbits induced by Sindbis virus. It did not inhibit the induction of an interferonlike inhibitor by bacterial endotoxin. It is suggested that the latter inhibitor is largely preformed, and its release does not require the transcriptive function of cell DNA. On the other hand, the production of at least some of the virus-induced interferon requires such function.