N‐acetylcysteine (NAC) is known to be a scavenger of free oxygen radicals, and recentin vitrostudies have demonstrated that it is also able to inhibit leukocyte function. The clinical significance of these effects is, however, not known. In this study we have measured the effect on human blood neutrophil and monocyte function of a single 400 mg dose of NAC administered orally. Administration of NAC to ten healthy volunteers resulted in significant reduction of neutrophil chemiluminescence response following activation by opsonized zymozan as compared to four non‐treated persons acting as controls. No effect was observed on the chemotaxis of either cell type or on monocyte chemiluminescence response. These findings suggest that NAC may be beneficial in clinical conditions like cystic fibrosis, where tissue damage may be a consequence of the effects of increased release of toxic oxygen radicals and proteolytic enzy