The in vivo effect of Escherichia coli-derived recombinant human granulocyte colony-stimulating factor on neutrophil function was studied in golden Syrian hamsters. Significant increases in superoxide generation and specific binding of N-formylmethionyl-leucyl-phenylalanine were observed in neutrophils isolated 4 h following a single subcutaneous injection of the factor (30 micrograms/kg). However, phagocytotic activity was not significantly stimulated in hamsters treated with the factor. Recombinant human granulocyte colony-stimulating factor hastened the recovery of peripheral neutrophil counts in animals made leukopenic by prior treatment with cyclophosphamide. Beginning several hours after infection, resistance to lethal infection following intraperitoneal injection of Staphylococcus aureus was increased when neutropenic animals were treated daily with the factor. This protective effect was associated with increased peritoneal neutrophil counts and a decreased incidence of positive peritoneal bacterial cultures at 24 h after the start of treatment. These results suggest that recombinant human granulocyte colony-stimulating factor may be a useful adjunct in the treatment of bacterial infections in neutropenic patients.