Interleukin-3 (IL-3), granulocyte-macrophage colony-stimulating factor (GM-CSF), and IL-5 are major hematopoietic cytokines produced by activated T cells and exhibit similar biologic activities by signaling through a common receptor subunit (beta c). Mice lacking beta c show a pulmonary alveolar proteinosis-like disease and reduced numbers of peripheral eosinophils, which are explained by the lack of GM-CSF and IL-5 function, respectively. However, beta c-deficient hematopoietic cells do respond to IL-3 normally, probably through an additional beta subunit of the IL-3 receptor (beta IL3) that is present in the mouse. Thus, almost normal hematopoiesis in beta c-deficient mice may be caused by functional redundancy between IL-3 and GM-CSF. To clarify the role of the entire IL-3/GM-CSF/IL-5 system in hematopoiesis in vivo, we crossed the beta c mutant mice with mice deficient for IL-3 ligand to generate mice lacking the entire IL-3/GM-CSF/IL-5 functions. The double-mutant mice were apparently normal and fertile. The severity of the lung pathology in the beta c/IL-3 double-mutant mice showed normal hemodynamic parameters except for reduced numbers of eosinophils and the lack of eosinophilic response to parasites, which were also found in beta c mutant mice. The immune response of the beta c/IL-3 double-mutant mice to Listeria mono-cytogenes was normal, as was hematopoietic recovery after administration of the cytotoxic drug, 5-fluorouracil. Although it has been believed that IL-3/GM-CSF/IL-5 produced by activated T cells play a major role in expansion of hematopoietic cells in emergency, our results indicate that the entire function of IL-3/GM- CSF/IL-5 is dispensable for hematopoiesis in emergency as well as in the steady state. Thus, there must be an alternative mechanism to produce blood cells in both situations.