Abstract
Semliki Forest virus, inactivated by incubation at 37°, induced considerably more interferon than did the live virus. Infection by live virus produced an earlier termination of interferon synthesis (at 8 hr) which correlated with a profound inhibition of cellular RNA synthesis. Heat-inactivated Semliki Forest virus did not inhibit cellular RNA synthesis until much later and permitted continuation of interferon synthesis for an additional 4 hr, thereby increasing the yield of interferon by about four- to sixfold.