FAS is a type-I membrane protein that transduces an apoptotic signal(1,2). Binding of Fas ligand or agonistic anti-Fas antibody to Fas kills the cells by apoptosis(3). Studies in the nematode Caenorhabditis elegans have suggested that proteases such as interleukin-1 beta-converting enzyme (ICE) or the product of the C. elegans cell-death gene ced-3 are involved in apoptotic signal transduction(4). The activity of ICE can be inhibited by the product of crmA, a cytokine-response modifier gene encoded by cowpox virus(5-7). We report here that expression of crmA inhibits cytotoxicity induced by anti-pas antibody or tumour necrosis factor (TNF). We have found a specific ICE inhibitor tetrapeptide (acetyl-Tyr-Val-Ala-Asp-chloromethylketone)(8,9) that also prevents apoptosis induced by anti-Fas antibody. These results suggest an involvement of an ICE-like protease in Fas-mediated apoptosis and TNF-induced cytotoxicity.