The p53 tumor suppressor protein is known to regulate the expression of the CD95 (Fas/APO-1) death receptor in a small subset of normal cell types as well as in many cancer cell types. However, whether p53-dependent regulation of CD95 expression is consistently associated with increased susceptibility to CD95-mediated cell death is poorly understood. To address this issue, we examined constitutive and induced CD95 surface expression and function in wild-type p53-expressing carcinoma cells relative to their isogenic p53-inactivated counterparts. We compared HCT116 colorectal carcinoma cells with their p53 biallelic knock-outs and control-transfected MCF-7 breast carcinoma cells with MCF-7 cells expressing a miniprotein inhibitor of p53 (p53DD). In both cell lines, the constitutive expression of surface CD95 was significantly reduced in p53-inactivated cells, as was the apoptotic response to agonistic anti-CD95 antibody. In both cell lines, only cells with wild-type p53 activity exhibited up-regulation of surface CD95 after ionizing irradiation. Interestingly, induction of CD95 expression substantially enhanced the apoptotic response to CD95 ligation only in MCF-7 cells but not in HCT116 cells. These findings provide direct evidence for a major role for wild-type p53 activity in regulating constitutive expression and function of CD95 in carcinoma cells; however, they also demonstrate that the functional effect of DNA damage-induced up-regulation of CD95 may be cell type specific.