We have used immunohistochemistry and monoclonal antibodies to analyze the phenotypic composition and activation status of the cellular infiltrate of bronchial biopsies obtained by fiber optic bronchoscopy of 11 atopic asthmatic subjects (FEV1 % predicted range 78 to 114), 9 atopic nonasthmatic control subjects, and 10 normal healthy subjects. Examination of mucosal biopsies obtained from both central (level I) and subsegmental (level II) bronchi showed that the highest number of CD45-, DC3-, DC4-, and CD8-positive cells were found in the group with asthma. There was a significant increase in the number of interleukin-2 receptor (CD25)-positive cells (a marker of lymphocyte activation) at airway level I in the asthmatic group compared with both nonasthmatic atopic (p < 0.05) and normal control subjects (p < 0.01). Eosinophil numbers were significantly increased in asthma at both airway levels and at airway level II in the nonasthmatic atopic group when compared with normal healthy control subjects (p < 0.05). EG2-positive cells (an index of secretion of eosinophil cationic protein following activation) were found at both airway levels in the asthmatic group and at level I in the nonasthmatic atopic control group (p < 0.05). When asthmatic subjects were compared with normal healthy subjects, there was a reduction in the number of neutrophil elastase-positive cells in the asthmatic subjects which, as a percentage of leukocytes, was significant (p = 0.05). When normoresponsive subjects were compared with hyperresponsive subjects (PC20 < 8 mg/ml methacholine), the hyperresponsive subjects had a significantly higher percentage of CD25-positive cells at airway level I (p < 0.01) and of EG2-positive cells at levels I and II (p < 0.01). These results support the view that the recruitment and “activation” of lymphocytes and eosinophils are important in the pathogenesis of bronchial asthma.