Summary When cells were pretreated with amounts of interferon which caused a 92 to 99% reduction in virus production, the yields of infectious RNA were inhibited by 67 to 81%, while the rate of incorporation of [3H]adenosine into total virus specific RNA was inhibited by only 48 to 65%. Sucrose gradient analysis of the virus specific RNA synthesized in untreated infected cells showed it to consist of (i) 45 S infectious RNA which is probably the RNA of the mature virus, (ii) 26 S ribonuclease-sensitive RNA, possessing not more than 3% of the infectivity of 45 S RNA, whose function is not known, but which is the major component early in infection, and (iii) 20 S ribonuclease-resistant RNA, which is probably a double-stranded replicative form. The inhibition by interferon of the synthetic rates of these three RNA species was not uniform. 45 S RNA showed the same sensitivity to interferon as did infectious RNA. The synthesis of 20 S ribonuclease-resistant RNA and of 26 S ribonuclease-sensitive RNA were relatively resistant to interferon action.