The oncogenes RAS and RAF came to view as agents of neoplastic transformation. However, in normal cells, these genes can have effects that run counter to oncogenic transformation, such as arrest of the cell division cycle, induction of cell differentiation, and apoptosis. Recent work has demonstrated that RAS elicits proliferative arrest and senescence in normal mouse and human fibroblasts. Because the Raf/MEK/MAP kinase signaling cascade is a key effector of signaling from Ras proteins, we examined the ability of conditionally active forms of Raf-1 to elicit cell cycle arrest and senescence in human cells. Activation of Raf-1 in nonimmortalized human lung fibroblasts (IMR-90) led to the prompt and irreversible arrest of cellular proliferation and the premature onset of senescence. Concomitant with the onset of cell cycle arrest, we observed the induction of the cyclin-dependent kinase (CDK) inhibitors p21^Cip1 and p16^Ink4a. Ablation of p53 and p21^Cip1 expression by use of the E6 oncoprotein of HPV16 demonstrated that expression of these proteins was not required for Raf-induced cell cycle arrest or senescence. Furthermore, cell cycle arrest and senescence were elicited in IMR-90 cells by the ectopic expression of p16^Ink4a alone. Pharmacological inhibition of the Raf/MEK/MAP kinase cascade prevented Raf from inducing p16^Ink4a and also prevented Raf-induced senescence. We conclude that the kinase cascade initiated by Raf can regulate the expression of p16^Ink4a and the proliferative arrest and senescence that follows. Induction of senescence may provide a defense against neoplastic transformation when the MAP kinase signaling cascade is inappropriately active.