Cysteine proteases of the interleukin 1 beta Converting Enzyme (ICE)/CED-3 family have been implicated in the effector process of apoptosis in several systems, including Fas-mediated apoptosis. We have recently isolated and partially characterized a protease present in extracts from anti-Fas antibody treated Jurkat T cells that promotes apoptotic changes in isolated nuclei (Schlegel, J., Peters, I., and Orrenius, S. (1995) FEBS Lett. 364, 139-142). We now show that this protease cleaves poly-(ADP-ribose) polymerase (PARP) with high efficiency and specificity. Both PARP proteolysis and the proapoptotic effects of the protease are inhibited by nanomolar concentrations of a selective inhibitor of apopain (CPP32), while an inhibitor of IL-1 beta converting enzyme is much less effective, requiring micromolar concentrations for the inhibition of the isolated protease. Kinetic analysis of the isolated protease reveals kinetic constants similar to those reported for apopain. The isolated protease is recognized by antibodies specific for CPP32/apopain but not by an anti-ICE antibody. Furthermore, a selective inhibitor of apopain prevents Fas-induced apoptosis in intact Jurkat T cells. We therefore conclude that CPP32/apopain is activated in Fas-induced apoptosis.