Alveolar macrophages (AM) appear to be one type of cells of the bronchial inflammation often involved in patients with asthma. These cells were demonstrated to generate highly reactive toxic species of oxygen in many pulmonary disorders; however, this was never demonstrated in asthma. The respiratory burst of bronchoalveolar lavage AM was studied in 12 subjects with asthma and in five control subjects by luminol-enhanced chemiluminescence (Cl). Both in subjects with asthma and in healthy subjects, the maximal value of Cl was obtained after 10 to 13 minutes of stimulation by opsonized zymozan. Baseline values and maximal Cl of AM values were significantly increased (p less than or equal to 0.03 and p less than or equal to 0.01) in subjects with asthma. There was a significant (p less than or equal to 0.01) correlation between maximal Cl of AM and the severity of asthma, assessed by a clinical score defined by Dr. K. Aas. The percentage of bronchoalveolar lavage eosinophils was significantly correlated (p less than or equal to 0.01) for all the subjects with the peak of CL of AM. This study suggests the role of toxic oxygen species in the bronchial inflammation in asthma.