Although granulocyte colony-stimulating factor (G-CSF) has been reported to act on cells of neutrophilic lineage, the expression of receptors for G-CSF (G-CSFR) on human hematopoietic progenitor cells has been unclear. We then analyzed the expression of G-CSFR on human bone marrow and G-CSF mobilized peripheral blood CD34+ cells, and examined the proliferation and differentiation capabilities of sorted CD34+ G-CSFR+ and CD34+ G-CSFR- cells using methylcellulose clonal culture. These results indicate that the expression of G-CSFR on CD34+ cells is restricted to myeloid progenitors, suggesting that the specific activity of G-CSF on myelopoiesis depends on the exclusive expression of its receptor on myeloid progenitors, and that the mobilization of various hematopoietic stem/progenitor cells is not a direct effect of G-CSF in humans.