Bronchial biopsies obtained from 16 asthmatic patients and six normal subjects were analyzed by immunohistochemistry. In the asthmatic patients, the total numbers of macrophages infiltrating the airway mucosa were increased. Many of the macrophages had the phenotypic characteristics of blood monocytes. HLA Class II antigen was expressed on infiltrating cells and airway epithelial cells. In biopsies from the asthmatics there was a significant increase in activated eosinophils, but not in neutrophils. There was also a significant increase in the numbers of T-lymphocytes in the asthmatics, but very few B-lymphocytes were detected. These results suggest that lung macrophages may have a central role to play in the mechanisms of the chronic immune-mediated inflammatory response seen in the airway mucosa of asthmatic patients.