The advances in virus research made possible by the use of amanitins are reported; they include i) determination of the genetic source (i.e. cellular or viral) of the RNA polymerase transcribing the viral genome, and ii) mechanism of virus-induced shut-off of host RNA synthesis. In the studies on the genetic origin of the RNA polymerase transcribing the viral genome, α-amanitin has been used mainly in cell-free systems because of its slow penetration intoin vitro cultured cells. Recently, amanitin penetration has been accelerated either by the use of derivatives of the toxin or by a short pretreatment of the cultures with DEAE-dextran. With the latter procedure it has been possible to test the action of amanitin on herpesvirus RNA synthesis in intact cells. α-amanitin has been used on the general assumption that the viral transcription inhibited by the toxin is carried out by the host polymerase B, although the existence of a virus-specific amanitin-sensitive transcriptase could not be excluded. Involvement of RNA polymerase B in herpesvirus and papovavirus transcription has been clearly demonstrated by comparative experiments with normal cells and mutant cells possessing amanitin-resistant RNA polymerase B; virus-coded transcriptase could be dismissed by the observation that amanitin did not inhibit viral RNA synthesis in the mutant cells. In the experiments concerning the mechanism of inhibition of host cell RNA synthesis, α-amanitin has been used to determine which RNA polymerase activity was inhibited during virus infection. Finally, in frog virus 3-infected cells direct titration of the number of RNA polymerase B molecules was performed by means of labeled amanitin. With this procedure it was confirmed that the drop in RNA polymerase B activity was due to a decreased number of enzyme molecules.