As the applications of hematopoietic growth factors increase, their complex impact on host defense and immune responses continues to unfold. The effect of the administration of granulocyte colony-stimulating factor (G-CSF) on bacterial defense, proliferation of lymphocytes, and cytokine production by lymphocytes and peripheral blood mononuclear cells (PBMC) was studied. The effect of G-CSF administration on the phenotype of the cells in the major hematopoietic organs was studied as well. ACI rats were given 10 mg/kg/day G-CSF or vehicle daily for 4 days. Isolated bone marrow neutrophils and enterocytes from treated animals showed a greater bactericidal activity than controls. Proliferation of mitogen-stimulated lymphocytes and PBMC was reduced in G-CSF-treated animals. The production of proinflammatory cytokines, tumor necrosis factor (TNF), and interleukin 6 (IL-6) by lymphocytes and PBMC was reduced by G-CSF pretreatment. G-CSF administration caused an increase in IL-4 (Th2 cytokine) release and a decrease in interferon-gamma (IFNgamma, Th1 cytokine) release by mitogen-stimulated lymphocytes. Cytometric analysis of cells in the progenitor cell region indicated a large increase in immature cells in the bone marrow of G-CSF-treated animals compared with sham along with an increase in B cells and a decrease in polymorphonuclear leukocytes (PMNs). In addition, cytometric analysis showed a large increase in PMNs in blood and splenocytes of the treated animals compared with sham. This study confirms and extends previous observations that G-CSF administration has a number of effects that might simultaneously enhance host defense while reducing the risk of developing uncontrolled systemic inflammation. This may also be efficacious in prolonging graft survival and reducing graft vs. host disease.