Hematopoietic progenitor cells (HPC) can be mobilized from the bone marrow into the peripheral circulation in response to diverse stimuli, including hematopoietic growth factors, cytotoxic agents, and certain chemokines. Despite significant differences in their biologic activities, these stimuli result in the mobilization of HPC with a similar phenotype, suggesting that a common mechanism for mobilization may exist. To explore the mechanisms of granulocyte colony-stimulating factor (G-CSF)-induced mobilization, we examined HPC mobilization in mice that are genetically deficient for the G-CSF receptor (G-CSFR). Their response was determined to each of three major types of mobilizing stimuli: cytotoxic agents (cyclophosphamide), chemokines (interleukin-8 [IL-8]), and hematopoietic growth factors (G-CSF, flt-3 ligand, and IL-12). These studies demonstrate that the G-CSFR is required for mobilization in response to cyclophosphamide and IL-8, but not flt-3 ligand or IL-12, and suggest that the G-CSFR may play an important and previously unexpected role in HPC migration. Copyright (C) 2000 by W.B. Saunders Company.