In increasing numbers of pulmonary diseases an association with a loss of intracellular thiols, mainly glutathione, is postulated. Therefore, the quantitative measurement of thiols within different viable cells is a possible metabolic parameter for cellular function and defense capacity of all pulmonary immune cells including alveolar macrophages (AM), that are highly compromised by oxidative stress. In this study the cellular thiol content was determined using fluorochrom conjugated chloromethyl derivatives (5-chloromethylfluorescein diacetate, CMFDA) in flow cytometry. The procedure was evaluated in vitro using biochemical techniques for glutathione quantification. Based on this approach, AM obtained from bronchoalveolar lavage (BAL) of smokers and patients with chronic obstructive pulmonary disease (COPD) showed a significant thiol deficiency compared to a nonsmoker/non-COPD group. The cellular thiol expression of AM from smokers and COPD patients reached only 50 and 53% of the control group. Lowest thiol concentrations (47% of control) were detected within the smoker(+)/COPD(+) group. This intracellular thiol deficiency significantly correlated with reduced lung function (FEV(1), PaO(2)). With regard to the tightly regulated thiol metabolism of immune cells, these results imply the onset of functional disturbances in thiol deficient AM. The determination of the cellular thiol content of AM, obtained from BAL by flow cytometry, presents a simple and reliable tool to monitor the effect of therapeutic measures focusing on the stabilization of the cellular thiol status.