Granulocyte colony-stimulating factor (G-CSF) stimulates proliferation of myeloid cells and maybe a valuable adjunct in prevention and treatment of neutropenia-associated infections. Neutrophil (PMNL)phagocytic and microbicidal functions against Staphylococcus aureus and Candida albicans blastoconidia were therefore evaluated. Bacterial phagocytosis and bactericidal activity were significantly enhanced by ∼ 50%–70% after preincubation of normal PMNLw ith G-CSF in concentrations of 1000–4000 units/ml for 10 min at 37°C. G-CSF in similar concentrations enhanced the defective bactericidal activity of PMNL from HIV-1-infected patients by ∼70%–150% and reached the baseline control PMNL killing. However, G-CSF enhanced neither phagocytosis nor fungicidal activity of normal PMNL against C. albicans blastoconidia. These data demonstrate that G-CSF enhances the antibacterial but not the antifungal activities of human PMNL in vitro and also improves the defective PMNLbactericidal activity of HIV-1-infected patients.