Polycomb group (Pc-G) proteins associate to form large complexes that repress Hox genes, thereby imposing Hox gene expression pattern required for development. However, Pc-G proteins have a Hox-independent function in controlling cell proliferation. Here we show that embryonic fibroblasts derived from M33-deficient mice are impaired in the progression into the S phase of the cell cycle, as shown by a reduced rate of incorporation of bromodeoxyuridine. These cells have a senescent phenotype, associated to an abnormal accumulation of the cyclin-dependent kinase inhibitor p16INK4a protein. We demonstrate that this defect is bypassed in mutant embryonic fibroblasts expressing a transdominant negative form of the cell cycle controlling transcription factor E2F (E2F-DB). In addition, we show that the polycomb protein M33 controls critical expansion of B- and T-lymphocyte precursors. Together, our results emphasize M33-Polycomb protein function in cell cycle control.