The cell-cycle inhibitor p21 is upregulated during senescence and upon induction of senescence-like arrest by oncogenic Ras. We have used primary fibroblasts derived from p21-null mice to evaluate the role of p21 in these processes. We find that primary p21−/− cells enter senescence and have a lifespan similar to wild-type cells. Upon immortalization, most wild-type and p21−/− cultures acquire alterations in either p53 or p16 INK4a, further indicating that p21-deficiency is not sufficient by itself to allow immortalization. Primary p21−/− cells, like wild-type cells, respond to oncogenic Ras by accumulating p53 and p16 INK4a, and by decreasing their proliferation rate. In agreement with this, p21−/− cells are refractory to neoplasic transformation by oncogenic Ras when compared to p53−/− cells. We conclude that, in murine fibroblasts, p21 is not essential neither for senescence nor for preventing neoplasic transformation by oncogenic Ras.