Granulocyte colony-stimulating factor (G-CSF) was reported to increase the number of colony-forming units-spleen (CFU-S) and multilineage colonies as well as myeloid-committed cells. We investigated the effects of G-CSF on myeloid progenitors and primitive stem cells in a mouse bone marrow transplantation (BMT) system. Lethally irradiated mice received BM cells from untreated or 5-fluorouracil-treated mice, and then were administered G-CSF or carrier buffer (control) for 5 days from immediately after BMT. A pre-CFU-S assay was performed by the repeated transplantation of BM cells from the first BMT recipients to other mice. By the method of polymerase chain reaction, most of the spleen colonies in the secondary recipients were confirmed to be derived from the first donors. G-CSF did not increase the peripheral white blood cell count significantly, but did increase the number of immature myeloid cells and granulocyte-macrophage colony-forming cells in the BM. The number of erythroid cells in the BM was initially suppressed and then increased by G-CSF treatment. In addition, the pre-CFU-S assay showed an increase in pre-CFU-S cells due to G-CSF administration. The number of spleen colonies of first BMT recipients did not increase, but a higher percentage of them were committed to a certain lineage by G-CSF treatment. These findings suggest that G-CSF has important roles in the early stages of hematopoiesis.