OBJECTIVE Whether G-CSF enhances the maturation of neutrophilic granulocytes or just accelerates the mobilization of mature and maturing granulocytes from bone marrow to blood, or both, is not clear. Using an in vivo culture system where such mobilization cannot take place, we previously showed that G-CSF did not accelerate maturation. To further clarify the role of G-CSF, we now have examined its effect on murine granulopoiesis in situ. METHODS Murine bone marrow precursors in S-phase were labeled with BrdU, and hematopoiesis stimulated by the long-acting G-CSF compound pegfilgrastim (peg-G-CSF). Performing flow cytometric analysis of incorporated BrdU and the granulocyte maturation antigen Gr1, we investigated the cell flux from the proliferative to the non-proliferative granulocyte compartments in bone marrow and further from bone marrow to blood. RESULTS Peg-G-CSF mobilized neutrophils from bone marrow to blood and markedly increased their concentration in blood for several days. It also increased the proliferation of precursor cells. Newly produced, less mature granulocytes (Gr1(+) BrdU(+)) travelled faster to blood in treated mice than in controls. The flow cytometric and cell density analyses of the bone marrow cells showed that peg-G-CSF skewed the population toward less mature cells, mainly because of the mobilization of granulocytes to blood. CONCLUSIONS Collectively, our data do not support the notion that G-CSF accelerates murine granulocyte maturation per se.