Concentrations of 0,8—90 μg α-amanitin per ml do not inhibit the growth of Agrobacterium tumefaciens in vitro. 20 ug/ml of the drug reduce tumor initiation by these bacteria (on leaves of Kalanchoe) if it is added simultaneously with the bacteria or 24 hours after infection. Addition of α-amanitin 96 hours after bacterial infection does not reduce the number of initiated tumors but it slows down tumor growth. These results demonstrate the participation in tumorigenesis of RNA synthesis in the nucleoplasm of the host cells.