Abstract DNA-directed RNA polymerase was solubilized from total HeLa cells. Three distinct classes of the enzyme could be clearly differentiated by their sensitivity toward α-amanitin. While form A is completely resistant to high concentrations (133 μg/ml) of this toxin, enzyme B is highly sensitive and is completely inhibited by concentrations of 0.1 μg/ml. In contrast, RNA polymerase C shows an intermediate behaviour (50% inhibition at 30% μg/ml). Separation of the three individual enzymes was achieved by chromatography on DEAE-cellulose (to separate enzyme B from A and C) and DEAE-Sephadex (to separate polymerase A from C). All three RNA polymerases were subsequently purified by phosphocellulose chromatography followed by sedimentation through glycerol gradients. Analysis of the purified enzymes by gel electrophoresis under denaturating conditions showed that the A enzyme consists of five subunits with molecular weights of 185, 128, 65, 41 and 32 × 10 3 . In contrast, polymerase B is composed of seven subunits in variable stoichiometry with molecular weights of 215, 175, 145, 123, 68, 43 and 31 × 10 3 respectively. The subunit structure of enzyme C is not entirely clear at present and remains to be established. In addition, RNA polymerase activities were solubilized from mitotic and middle-S phase cells in comparison to controls. With respect to amounts and/or activities of all three RNA polymerases A,B and C no significant differences were detectable between logarithmically growing, mitotic and middle-S phase cells.