Inhibitory and stimulatory cytokines regulate the function and survival of hematopoietic progenitor cells. Interactions between cytokines and progenitor cells may result in programmed cell death. Apoptosis of hematopoietic cells ultimately serves to diminish the size of the stem cell compartment in bone marrow (BM) failure, and this has frustrated efforts at ex vivo expansion of hematopoietic stem cells for BM transplantation. We previously demonstrated that triggering of the Fas-receptor, which is expressed on BM CD34+ cells, mediates apoptosis of progenitor cells. Although interleukin-1beta-converting enzyme (ICE) appears to be an important factor in the signaling cascade regulating Fas-mediated apoptosis of lymphoid cells, its role in apoptosis of CD34+ cells has not been explored. In this study, we determined whether ICE protein was present in CD34+ cells and assessed its role in limiting expansion of hematopoietic stem cells by apoptosis. We demonstrated that ICE mRNA was constitutively produced in CD34+ cells, although the active form of ICE protein was not detected in fresh, unstimulated CD34+ cells from healthy donors. ICE protein could be induced in these CD34+ cells when they were cultured for 24 hours in the presence of hematopoietic growth factors. Interferon (IFN)-gamma and Fas agonist (CH11 monoclonal antibody) enhanced ICE expression and triggered CD34+ cell apoptosis and cell death. In both short- and long-term BM cultures, hematopoietic colony-forming cell numbers were increased after ICE blockade with a synthetic ICE inhibitor (Ac-Tyr-Val-Ala-Asp-aldehyde), even in the absence of IFN-gamma, suggesting that ICE regulates the proliferation and cell death of committed and primitive progenitor cells. The suppressive effect of IFN-gamma and Fas agonist on colony formation was also antagonized by ICE inhibitor. The effects of ICE blockade on proliferation of hematopoietic progenitors cells were related to inhibition of apoptosis, as demonstrated by annexin staining and in situ terminal dideoxytransferase apoptosis assays. Our results suggest that ICE protein is present in CD34+ cells after exposure to cytokines, that regulation of the levels of ICE protein in CD34+ cells is posttranscriptional, and that ICE plays a role in the regulation of apoptosis and expansion of primitive hematopoietic cells. ICE inhibition could potentially be used to reverse intrinsic and cytokine-mediated apoptotic signals for the purpose of stem and progenitor cell expansion.