The production of superoxide radicals (O2-) by human granulocytes after phagocytosis of latex particles has been studied as a function of cell, latex, and cytochrome c concentration. Under the chosen conditions, latex phagocytosis stimulated the reduction of cytochrome c by O2- sevenfold to 420 mumoles per 10-10 granulocytes per hour. At high cytochrome c concentrations, this value increased to 800 to 1,000 mumoles per 10-10 granulocytes per hour. These amounts suggest that O2- plays an important role in the formation of H2O2 during the phagocytic process. We propose that the extra oxygen consumed in the respiratory burst is first reduced to O2-, possibly by ascorbate, before being converted to H2O2. No stimulation of the superoxide radical formation was found in the phagocytizing granulocytes of two patients with chronic granulomatous disease, and intermediate values were found for the heterozygotes of this deficiency.