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A hamster cell line resistant to alpha-amanitine has been isolated (alpha-am-r, BHK-T6-G-1). Cell extracts of this mutant have an alpha-amanitine-resistant RNA polymerase II (nucleosidetriphosphate: RNA nucleotidyl-transferase, EC 2.7.7.6) activity as shown by DEAE-cellulose column chromatography. This mutation is dominant in interspecific hybrids with 3T3 mouse cells. In such hybrids polyoma virus can grow with equal efficiency in the presence or absence of the drug, thus indicating that the RNA polymerase of the unsusceptible parental cell can participate in the correct transcription of the viral genome.