The objective of this study was to characterize the role of Bcl-2 expression in Fas-mediated apoptosis in human renal cell carcinoma (RCC) cell lines. RT-PCR analyses showed that 10 different RCC cell lines expressed Fas, but not Fas ligand. Seven of 10 cell lines expressed Fas strongly, while 3 cell lines weakly expressed Fas by flow cytometric analyses. Measurement of the LDH concentration in the culture supernatant revealed that 6 of the 7 cell lines which expressed Fas strongly were sensitive to treatment with an agonistic anti-Fas monoclonal antibody (CH11), whereas all cell lines which weakly expressed Fas did not show Fas-mediated cell death. Furthermore, Bcl-2 expression was found only in three cell lines which were all susceptible to CH11, and the downregulation of Bcl-2 protein by treatment with antisense oligodeoxynucleotide targeting Bcl-2 gene resulted in an enhancement of cell death induced by CH11. These findings suggest that strong Fas expression is necessary for Fas-mediated cell death in RCC cell lines, and Bcl-2 has a protective role against treatment with an anti-Fas monoclonal antibody, despite the fact that Bcl-2 expression was observed only in sensitive cell lines to Fas-mediated cell death.