Disorders in negative growth control by CD95 (APO-1/Fas)-mediated apoptosis have been suggested to facilitate immune evasion of neoplastic cells and resistance to anticancer drugs. The present report describes the expression and function of CD95 receptor and ligand in human renal cell carcinomas (RCC) of all major histological types, presenting in vitro data on RCC cell lines (n = 30) and ex vivo observations in RCC tissue samples (n = 30). Using RT-PCR, flow cytometry and immunostaining, expression of CD95 receptor and ligand was found in human RCC of all major histological types. The expression levels of CD95 ligand, however, were rather low. Despite constitutive CD95 receptor expression, resistance to CD95-mediated apoptosis became evident from the weak response to agonistic anti-CD95 antibodies, which induced a low increase of apoptosis in only 9 of 30 RCC cell lines. After IFN-gamma pretreatment, however, apoptosis triggered by agonistic anti-CD95 antibodies was observed in 22 of 30 RCC cell lines. These data indicated that the machinery required for CD95-induced cell death was present, albeit inactive in most RCC. Inhibition of protein synthesis by cycloheximide resulted in increased sensitivity to agonistic anti-CD95 antibodies, suggesting a role for short-lived apoptosis-protective proteins in the resistance of RCC to CD95-mediated apoptosis. Moreover, we demonstrated the secretion of soluble CD95 receptor which might contribute to this resistance as well. In conclusion, resistance rather than responsiveness to CD95-mediated apoptosis is a key feature of human RCC. This resistance might facilitate evasion from negative growth control and contribute to the failure of cytotoxic drugs in the treatment of human RCC.