We propose a new role of retinoblastoma protein as a cell growth activator in its phosphorylated form. The hyper-phosphorylated retinoblastoma protein generated by the action of cdk2/cyclin E strongly stimulated the activity of DNA polymerase alpha, but did not stimulate DNA polymerases delta, epsilon, or primase. But, cdk4/cyclin D-phosphorylated retinoblastoma protein showed little stimulation. Hyper-phosphorylated retinoblastoma protein interacted with the catalytic subunit of DNA polymerase alpha, and stabilised DNA polymerase alpha from heat inactivation at 45 degrees C. These results suggest that in G1 phase, hypo-phosphorylated retinoblastoma protein suppresses the progression of cell cycle as a transcription inhibitor, but that after phosphorylation by cdk2/cyclin E at the G1/S boundary, hyper-phosphorylated retinoblastoma protein acts as a cell-cycle promoter by optimising the DNA polymerase alpha reaction.