Interleukin-1 beta (IL-1 beta) converting enzyme (ICE) is a cysteine protease that specifically cleaves precursor IL-1 beta to its biologically active form. Recent studies have also implicated ICE in the induction of apoptosis in vertebrate cells. Because IL-1 plays a major role in acute myelogenous leukemia (AML) blast proliferation, we sought to investigate the effect of ICE inhibition on AML progenitors. To do this, we used bocaspartyl (benzyl) chloromethylketone (BACMK) an inhibitor designed to penetrate cells and bind covalently to the active site of ICE. Our preliminary experiments showed that incubation of activated peripheral blood cells with 2.5 mumol/L of BAMCK downregulated production of mature IL-1 beta but had no effect on tumor necrosis factor-alpha. To test the effects of the inhibitor on AML cells, we first used the OCI/AML3 cell line. We found that these cells produce IL-1 beta and bind the biotinylated cytokine and that IL-1 inhibitors, such as IL-1 neutralizing antibodies, IL-1 receptor antagonist, and soluble IL-1 receptors, specifically inhibit OCI/AML3 proliferation, indicating that IL-1 beta is an autocrine growth factor for OCI/AML3 cells. The ICE inhibitor suppressed OCI/AML3 growth in a dose-dependent manner (at 0.4 to 4 mumol/L) and downregulated mature IL- 1 beta production, as assessed by Western immunoblotting. Similar results were obtained with marrow aspirates from 16 AML patients. The ICE inhibitor suppressed proliferation of AML precursors (by up to 78%; mean, 44%) in a dose-dependent fashion at concentrations ranging from 0.4 to 5 mumol/L but not proliferation of normal marrow progenitors; the suppressive effect was reversed by IL-1 beta. Furthermore, incubation of AML cells with 4 mumol/L BAMCK downregulated the production of mature IL-1 beta, suggesting that the growth-inhibitory effect is mediated through suppression of the biologically active cytokine. Our data indicate that inhibition of ICE suppresses AML blast proliferation and suggest that ICE inhibitors may have a role in future therapies for AML.