Six healthy volunteers underwent bronchoalveolar lavage (BAL) before and after receiving N-acetylcysteine (NAC) 600 mg daily for 2 weeks. Free and total NAC, cysteine and glutathione were determined in the lavage fluid, lavage cells and plasma. No NAC was demonstrated, free or bound in disulfides, in either of the lavage components; furthermore, the cysteine and glutathione content of these components and their respective redox states were unaltered during therapy. Plasma free and total cysteine content was unaltered by administration of the drug, but both free and total plasma glutathione increased significantly. Free NAC could not be detected in plasma following dosing. However, a mean of 0.3 nmol/100 microliters plasma was released from disulfides in plasma following reduction with dithiothreitol. N-acetylcysteine has been proposed to act as a mucolytic by cleavage of disulfide bonds. Our findings do not support this direct mode of action and alternative mechanisms of action must be sought.