G-CSF works through cells of the monocyte-macrophage lineage to suppress B lymphopoiesis. (A) Wild-type mice were treated with G-CSF (250 µg/kg per day) or saline alone for the indicated time, and the B cells in the bone marrow were quantified; 7P, 7 days after stopping G-CSF (n = 3-12). (B-C) Mixed chimera mice were generated by transplanting wild-type (Ly5.1/Ly5.2) and Csf3r^−/− (Ly5.2) bone marrow cells into irradiated wild-type (Ly5.1) recipients at a 1:2 ratio. Eight weeks after transplantation, mice were treated with G-CSF for 5 days or left untreated, and (B) B cells and (C) neutrophils were quantified by flow cytometry (n = 4-5). (D) CD68:Csf3r, Csf3r^−/− mice were treated for 5 days with phosphate-buffered saline (PBS) or G-CSF. Shown is the number of B cells in the bone marrow for each B-cell subset (n = 7-8). (E) CD68:Csf3r, Csf3r^−/− femurs were flushed with Trizol to collect total bone marrow RNA. Shown is the messenger RNA (mRNA) expression of the indicated gene relative to β-actin mRNA (n = 7-8). *P < .05; ***P < .001. ns, not significant.