Lung cells recovered from symptomatic patients with asthma generate increased amounts of reactive oxygen species (ROS). Animal and in vitro studies indicate that ROS can reproduce many of the features of asthma. The ability of ROS to produce the clinical features of asthma may depend on an individual's lung antioxidant defenses. Patients with asthma are reported to have reduced antioxidant defenses in peripheral blood, but little is known about the antioxidant defenses of their lung cells. To define lung cell antioxidant defenses in asthma, the glutathione concentration and the glutathione reductase, glutathione peroxidase, catalase, and superoxide dismutase (SOD) activities were measured in cells recovered by bronchoalveolar lavage (BAL cells) and by bronchial brushing (bronchial epithelial cells, HBEC) from normal subjects and patients with asthma. Superoxide dismutase activity was reduced 25% in BAL cells (p < .05) and nearly 50% in HBEC (p < .02) from patients with asthma. Alterations in the other antioxidants were not identified. A direct relationship was found between airway reactivity to methacholine, measured as PC(20)FEV(1), and HBEC SOD activity (r2 = 89; p < .005), but not between airway reactivity and the other antioxidants. The finding of reduced SOD activity in lung cells of patients with asthma suggests that diminished SOD activity serves as a marker of the inflammation characterizing asthma. Alternatively, it may play a role in the development or severity of the disease.