Activated cytolytic effector cells like lymphokine-activated killer (LAK) and the recently described bacillus-Calmette-Guerin-activated killer (BAK) cells are thought to mediate antitumor effects against metastatic renal cell carcinoma (RCC) and superficial bladder cancer respectively. Perforin and Fas ligand (FasL) have been described as the major lytic principles in cellular cytotoxicity. Using a radioactive-release assay and specific inhibitors, we investigated the molecular mechanisms used by LAK and BAK cells in the lysis of renal carcinoma cells. In addition, we evaluated the susceptibility of RCC cells to FasL-mediated cytotoxicity. LAK and BAK cells effectively lysed the renal cancer cell line SK-RC-35 upon cell-cell contact. Both effector cell populations were shown to produce perforin and FasL as determined by reverse transcriptase/polymerase chain reaction (RT-PCR). Using fluorescence-activated cell sorting analyses and RT-PCR, we detected a marked Fas receptor (Fas, CD95) expression on RCC cells. However, RCC cells were shown to be resistant to killing by recombinant FasL and lysis by BAK and LAK cells was not inhibited in the presence of anti-FasL antibody. In contrast, the cytotoxicity exerted by LAK and BAK cells was drastically reduced in the presence of the Ca2+-chelating agent EGTA as well as concanamycin A, a specific inhibitor of perforin-mediated lysis. These results demonstrate that cytolysis of FasL-resistant RCC cells by activated immune cells is mediated via perforin. Our findings give further insights into the molecular mechanisms involved in the elimination of RCC by cytotoxic lymphocytes activated with biological response modifiers.