Actinomycin D (5 γ/ml) inhibits the synthesis of fowl plague virus RNA, S-antigen, hemagglutinin, neuraminidase and infectious particles. Even high concentrations (40 γ/ml) of the antibiotic do not inhibit the synthesis of the corresponding components of NDV. Although purified suspensions of these viruses can be inactivated by actinomycin in the presence of light, this photoeffect does not play an important role in the inhibition of fowl plague virus synthesis. If in the case of fowl plague virus actinomycin is added to tissue cultures at a time after infection when viral RNA synthesis has already started (1½—2 hours p. i.) further synthesis of viral RNA is stopped, while the S-antigen titer continues to rise.