IL-1 beta-converting enzyme (ICE) cleaves pro-IL-1 beta to generate mature IL-1 beta. ICE is homologous to other proteins that have been implicated in apoptosis, including CED-3 and Nedd-2/lch-1. We generated ICE-deficient mice and observed that they are overtly normal but have a major defect in the production of mature IL-1 beta after stimulation with lipopolysaccharide. IL-1 alpha production is also impaired. ICE-deficient mice are resistant to endotoxic shock. Thymocytes and macrophages from the ICE-deficient animals undergo apoptosis normally. ICE therefore plays a dominant role in the generation of mature IL-1 beta, a previously unsuspected role in production of IL-1 alpha, but has no autonomous function in apoptosis.