Fas-induced apoptosis, a key event in the regulation of lymphocyte homeostasis, is mainly mediated by the activation of a cascade of caspases. Using caspase- and Fas-associated death domain protein-deficient Jurkat cell lines as well as a pan-caspase inhibitor (Z-VAD-fmk), we observed a second Fas-induced cell death event, independent of any known caspase activation. This pathway is of slow kinetics and displays some features of necrosis. However, this caspase-independent pathway does not seem to play a significant role in the Fas-mediated death of primary activated T cell blasts. Indeed, in this setting, Fas-induced cell death was always substantially inhibited by Z-VAD-fmk, suggesting that caspase activation is an absolute requirement in the Fas-induced death of primary human T lymphocytes.