Macrophage colony-stimulating factor (recombinant human M-CSF) given as a single intravenous injection to Lewis rats induces a dose-dependent peripheral monocytosis, neutrophilia, and lymphopenia. The monocytosis peaks at 28 to 32 hours with a seven- to eightfold increase in the number of circulating monocytes and promonocytes. The peripheral monocytosis is accompanied by a slight increase in marrow blasts, promonocytes, and monocytes. A monocytopenia reaching a nadir at 15 minutes precedes the monocytosis, suggesting that M-CSF activates circulating monocytes and causes intravascular margination. The M-CSF- induced neutrophilia and lymphopenia are relatively mild in magnitude, are observed between 2 and 16 hours after injection, and are no longer evident at later time-points. The monocytosis was at least partially inhibited by dexamethasone. M-CSF-induced monocytosis most likely reflects a direct effect of M-CSF on marrow monocyte precursor proliferation, maturation, and release, whereas the neutrophilia and lymphopenia may reflect indirect effects mediated by the known ability of M-CSF to cause the release of other cytokines.