The production of free oxygen radicals by polymorphonuclear cells (PMNs) was studied in 25 patients after blunt trauma. Superoxide generation significantly increased immediately after trauma and returned to normal soon after the event. Patients were subsequently divided into two groups: Those who developed sepsis and those who did not develop infectious complications. Superoxide production by intact PMNs following stimulation by three different stimulants was initially not different in trauma patients who developed sepsis. Follow-up showed an increase in superoxide production when infection complicated the course of trauma patients. Further studies were performed in a cell-free system containing cell membranes and cytosol from patients or healthy controls. No difference in the production of superoxide was found when membranes from trauma patients or controls were mixed with cytosols from controls. When cytosols from patients were mixed with membranes from controls, a significant increase in superoxide production was observed in the group that developed sepsis. Immunoblotting analysis of two protein components of the cytosolic portion of the NADPH oxidase, p47 and p67, were done. The increase in quantity of p47 correlated with the increase in superoxide production during sepsis, and thus may be the major contributor to the high activity.