The bmi-1 gene was first isolated as an oncogene that cooperates with c-myc in the generation of mouse lymphomas(1,2). We subsequently identified Bmi-1 as a transcriptional repressor belonging to the mouse Polycomb group(3-6). The Polycomb group comprises an important, conserved set of proteins that are required to maintain stable repression of specific target genes, such as homeobox-cluster genes, during development(7-9). In mice, the absence of bmi-1 expression results in neurological defects and severe proliferative defects in lymphoid cells, whereas bmi-1 overexpression induces lymphomas(4,10). Here we show that bmi-1-deficient primary mouse embryonic fibroblasts are impaired in progression into the S phase of the cell cycle and undergo premature senescence. In these fibroblasts and in bmi-1-deficient: lymphocytes, the expression of the tumour suppressors p16 and p19(Arf), which are encoded by ink4a, is raised markedly. Conversely, overexpression of bmi-1 allows fibroblast immortalization, downregulates expression of p16 and p19(Arf) and, in combination with H-ras, leads to neoplastic transformation. Removal of ink4a dramatically reduces the lymphoid and neurological defects seen in bmi-1-deficient mice, indicating that ink4a is a critical in vivo target for Bmi-1. Our results connect transcriptional repression by Polycomb-group proteins with cell-cycle control and senescence.