Monokines may contribute to the regulation of hematopoiesis and circulating numbers of leukocytes during chronic inflammation. The hematologic effects of daily intravenous injection of the recombinant monokines tumor necrosis factor (TNF), interleukin-1 (IL-1), and granulocyte-colony stimulating factor (G-CSF) were therefore studied in the bone marrow and circulation of rats over the course of a week. TNF induced daily neutrophilia and lymphopenia with no evidence of tachyphylaxis. TNF also induced a slight decrease in early myeloid forms in the marrow, but, more strikingly, induced a marked erythroid hyperplasia of late normoblasts, although no changes other than a slight reticulocytosis were noted in the peripheral red blood cell compartment. IL-1 also induced daily neutrophilia and lymphopenia with no evidence of tachyphylaxis. IL-1 differed from TNF in the induction of a significant myeloid hyperplasia and in the lack of any effect on the erythroid elements of the marrow. The lack of tachyphylaxis to the chronic administration of both TNF and IL-1 suggests that the mechanism of endotoxin-induced tachyphylaxis is not at the level of the effector cell response to these endogenous cytokines. G-CSF induced a biphasic peripheral neutrophilia first peaking on day one, reaching a nadir on day 4, and then rising progressively again until day 7. The low level of neutrophilia on day 4 is not due to marrow depletion of neutrophils secondary to the neutrophil releasing activity of G-CSF because the marrows of G-CSF-treated rats on both days 3 and 7 contained over twice the number of mature neutrophils as controls. Thus, the trough in the neutrophilia induced by G-CSF is postulated to be due to an as-yet unidentified negative feedback mechanism that inhibits neutrophil release from the marrow.