Two subforms of RNA polymerase III - IIIa and IIIb - were identified in human placenta nuclei These subforms differed in molecular weight of one subunit, and in buoyant density in glycerol concentration gradient. Protein kinase activity, which phosphorylates at least four subunits of RNA polymerase IIIa and three subunits of RNA polymerase IIIb in vitro, was copurified with both the subforms. Protein kinase activity was inhibited by wortmannin, a specific PI3-kinase inhibitor. RNA polymerase III dephosphorylation by alkaline phosphatase in vitro decreased the transcription level on specific Alu-template. The associated protein kinase was not able to phosphorylate dephosphorylated RNA polymerase IIIa and to restore the transcription level up to the control one.