The tumor suppressor p53 can induce growth arrest and cell death via apoptosis in response to a number of cellular stresses. We have shown previously that the immunosuppressant cyclosporin A (CsA) induces programmed cell death with typical features of apoptosis in rat glioma cells. We report that CsA treatment results in increased level of the p53 tumor suppressor, its nuclear accumulation, and transcriptional activation of p53-dependent genes. The increase of p53 correlates with the elevation of p21(Waf1) and Bax protein expression. The increased level of Bax protein was accompanied with changes in its subcellular localization and association with mitochondria. Importantly, we demonstrate that glioma cells stably transfected with a mutant p53 (p53Val135) fail to increase p21 and Bax protein levels and are less sensitive to CsA-induced apoptosis. Furthermore, primary fibroblasts from p53-/- knockout mice are significantly more resistant to CsA-induced apoptosis compared with their corresponding counterparts containing functional p53. Together, our results suggest that the apoptotic program activated by CsA can be mediated by activation of p53 tumor suppressor and potentiation of its ability to initiate apoptosis.