Interleukin (IL)-1 is a proinflammatory cytokine that plays a pivotal role in driving the in vitro proliferation of leukemic cells through autocrine or paracrine pathways. Both IL-1 genes, IL-1 alpha and the prominent IL-1 beta, produce 31 kDa proteins. Whereas the precursor (pro) 31 kDa form of IL-1 alpha is biologically active, pro-IL-1 beta is inactive unless cleaved to its mature form by a cytoplasmic cysteine protease termed IL-1 beta converting enzyme (ICE). Although ICE was first thought to be a unique enzyme with a single biologic activity, several investigators have demonstrated that ICE shares sequence homology with the protein product of ced-3, the gene for cell death of the nematode Caenorhabditis elegans, and can induce apoptosis in different cellular systems. However, recent data indicate that ICE is a member of an increasingly recognized family of ICE-related molecules whose other members, such as CPP32, do not cleave pro-IL-1 beta but rather are effective inducers of apoptotic cell death. We recently investigated the effect of ICE inhibition on acute myelogenous leukemia (AML) colony growth. We found that inhibition of ICE reduced the production of mature IL-1 beta and suppressed the proliferation of AML colony-forming units, confirming the central role of IL-1 beta in AML progenitor proliferation. These data suggest that the primary role of ICE in AML cells is cleavage of pro-IL-1 beta rather than induction of apoptosis and that the antileukemic activity of specific ICE inhibitors warrants further exploitation.