Purified Semliki Forest virus (SFV) contains three structural proteins while its core (nucleocapsid) contains two of these proteins. To identify all of the proteins synthesized under virus direction, cells were infected with SFV in the presence of actinomycin D and guanidine. Cell protein synthesis was markedly and irreversibly inhibited under these conditions; virus growth was reversibly inhibited by guanidine and began when the cells were washed to remove the guanidine. When cells were treated with guanidine for 4 hr after virus infection and then were washed, five major proteins were produced early in infection. Three of these proteins corresponded to virus structural proteins. None of these five proteins was a major protein of uninfected cells or of virus-infected cells which had been incubated with partially purified interferon before infection. Late in infection, three major proteins, the virus structural proteins, were produced.