We examined the rate and quantity of superoxide anion (O2-) generation by granulocytes harvested from the blood of patients before and after a 12-24 h constant intravenous infusion with recombinant human granulocyte-macrophage colony-stimulating factor (GM-CSFrh). Seven previously untreated patients with sarcomas who exhibited no bone marrow involvement by their disease were studied. Granulocytes were harvested from the peripheral blood of each patient before and after intravenous infusion with GM-CSFrh. The total quantity of O2- produced and the kinetics of its release were then determined following in vitro activation of the cells by the chemotactic peptide N-formyl methionyl-leucylphenylalanine (fMLP) or by phorbol myristate acetate (PMA). In all cases, the amount of O2- generated and the rate of its release were significantly increased after GM-CSFrh infusion. Our findings indicate that intravenous administration of GM-CSFrh to patients heightens the functional responsiveness of circulating granulocytes.