Oncogenic Ras induces two products of the INK4a/ARF tumor suppressor locus (p16(INK4a) and p19(ARF)) in primary human and rodent fibroblasts, ultimately leading to a permanent state of cell cycle arrest resembling replicative senescence. Whereas p16(INK4a) antagonizes the activities of cyclin D-dependent kinases, p19(ARF) activates the p53 transcription factor. Immortalized rodent fibroblast cell lines that lack INK4a/ARF function, ARF alone, or p53 are resistant to the growth inhibitory effects of oncogenic Ras and instead continue to proliferate and undergo morphological transformation. Primary mouse embryo fibroblasts lacking Cip1 and Kip1 genes encoding inhibitors of cyclin-dependent kinase-2 were used to further explore the effects of oncogenic Ras on arrest of the cell division cycle. Although early passage primary fibroblast strains that lack both p21(Cip1) and p27(Kip1) fail to assemble cyclin D-dependent kinases, oncogenic Ras retained its ability to induce p19(ARF), but not p16(INK4a), protecting Cip/Kip-null cells from proliferating and undergoing transformation. Under these conditions, Ras did not induce G1 phase arrest but instead triggered DNA synthesis, abnormal nuclear divisions, failure of cytokinesis, and emergence of polyploid cells. Therefore, in the absence of p16(INK4a), p21(Cip1), and p27(Kip1), oncogenic Ras affects the functions of genes required for completion of the cell cycle.