The effect of strenuous aerobic exercise on the liberation of reactive oxygen species production by human granulocytes was investigated in 9 untrained healthy men. The influence of supplementation with N-acetylcysteine, a pro-glutathione free radical scavenger, on this was also investigated. Subjects performed two identical maximal exercise tests (mean time 14.06 +/- SD 0.84 min) on a bicycle ergometer. Venous blood samples were taken before and after maximal exercise, with and without N-acetylcysteine supplementation (total dose 2400 mg within three days). Isolated granulocytes were stimulated in vitro with phorbol myristate acetate or zymosan particles coated with autologous serum. The production of reactive oxygen species was measured by luminol-enhanced chemiluminescence. An enhancement (priming) by 52% in the production of reactive oxygen species, as measured by the AUC, occurred after exercise when zymosan was used as stimulant. This priming effect was abolished following N-acetylcysteine supplementation. In contrast, phorbol myristate acetate stimulation did not show any influence of exercise in the chemiluminescence reaction without or with N-acetylcysteine supplementation. In conclusion, priming of granulocytes could be induced by a relative short bout of exercise. The priming of granulocytes due to exercise could be measured in this study by zymosan-stimulated luminol-enhanced chemiluminescence.