Programmed cell death or apoptosis provides an irreversible mechanism for the elimination of excess or damaged cells. Several recent studies have implicated the activation of the interleukin 1β-converting enzyme/Ced-3 (ICE/Ced-3) family of proteases as the “point of no return” in apoptotic cell death, while others have suggested that loss of mitochondrial membrane potential (ΔΨm) is the ultimate determinant of cell death. The temporal relationship of these two events during apoptosis and the role of Bcl-2 proteins in inhibiting these steps has not been defined. To examine these issues, control and Bcl-xL-transfected Jurkat T cells were treated with Fas antibodies in the presence and absence of the ICE protease inhibitor zVAD-FMK. ICE/Ced-3 protease activity was monitored by following the cleavage of poly(ADP-ribose) polymerase (PARP) and ΔΨm was followed by rhodamine 123 fluorescence. Although Bcl-xL expression did not block Fas-induced protease activation, it substantially inhibited the subsequent loss of ΔΨm and cell death in Fas-treated cells. In contrast, zVAD-FMK blocked PARP cleavage as well as loss of ΔΨm and cell death. Together these data demonstrate that Bcl-xL can maintain cell viability by preventing the loss of mitochondrial membrane potential that occurs as a consequence of ICE/Ced-3 protease activation.