The effect of recombinant human granulocyte colony-stimulating factor (rhG-CSF) on neutropenia was studied in 30 patients with aplastic anemia (AA). RhG-CSF was administered intravenously daily at a dose of 2, 5, 10, or 20 micrograms/kg/day for more than 7 days. In the patients whose absolute neutrophil counts (ANC) were more than 0.1 X 10(9)/l, the rhG-CSF injections at greater than or equal to 5 micrograms/kg/day caused rapid and selective elevation of ANC which maintained during the injection period. Most of the patients were well tolerated, and minor side effects were observed in only 3 patients. These findings suggest that daily injections of rhG-CSF at a dose of greater than or equal to 5 micrograms/kg/day may be an effective strategy for the treatment of bacterial and/or fungal infections in AA patients.