The function of the p53 tumor suppressor protein must be highly regulated because p53 can cause cell death and prevent tumorigenesis. In cultured cells, the p90 MDM2 protein blocks the transcriptional activation domain of p53 and also stimulates the degradation of p53. Here we provide the first conclusive demonstration that p90 MDM2 constitutively regulates p53 activity in homeostatic tissues. Mice with a hypomorphic allele of mdm2 revealed a heretofore unknown role for mdm2 in lymphopoiesis and epithelial cell survival. Phenotypic analyses revealed that both the transcriptional activation and apoptotic functions of p53 were increased in these mice. However, the level of p53 protein was not coordinately increased, suggesting that p90 MDM2 can inhibit the transcriptional activation and apoptotic functions of p53 in a manner independent of degradation. Cre-mediated deletion of mdm2 caused a greater accumulation of p53, demonstrating that p90 MDM2 constitutively regulates both the activity and the level of p53 in homeostatic tissues. The observation that only a subset of tissues with activated p53 underwent apoptosis indicates that factors other than p90 MDM2 determine the physiological consequences of p53 activation. Furthermore, reduction of mdm2 in vivo resulted in radiosensitivity, highlighting the importance of mdm2 as a potential target for adjuvant cancer therapies.