Fas ligand and perforin are the two key effector mechanisms in T cell-mediated cytotoxicity. These molecules mediate cytolysis of target cells by membrane damage and apoptosis. bcl-2 is known to protect cells against apoptosis induced by many stimuli including growth factor removal. However bcl-2s effect on Fas ligand and perforin-induced lysis has not been studied extensively. We investigated the effect of overexpression of bcl-2 alone, Fas alone or their combined overexpression on lysis of a commonly used target, P815, by perforin-sufficient, Fas ligand-sufficient and perforin-deficient or Fas ligand-deficient, allospecific cytotoxic T lymphocytes (CTL). Wild-type P815 are susceptible to lysis by perforin-sufficient CTL, regardless of the presence or absence (gld) of Fas ligand, but are poorly lysed by perforin-deficient CTL. Fas transfection of P815 makes target cells highly susceptible to lysis by both perforin-sufficient and -deficient CTL, indicating the presence of the Fas ligand-mediated cytotoxicity on both types of CTL. Co-transfection of P815-fas with bcl-2 abolishes their increased susceptibility to Fas-mediated lysis, even in the face of Fas overexpression on the cell membrane. The protective effect of bcl-2 against cell lysis is evident with perforin-deficient CTL as effector cells or when perforin activity is eliminated by the absence of extracellular calcium in perforin-sufficient CTL. bcl-2 overexpression by P815, however, does not protect against CTL lysis by the perforin pathway, regardless of Fas overexpression, as demonstrated by Fas ligand mutated gld and wild-type perforin-sufficient CTL. Therefore bcl-2 can protect P815 target cells against Fas-mediated lysis when triggered by the Fas ligand on CTL, but not against perforin-mediated lysis.