The eosinophil granule major basic protein (MBP) is toxic to parasites and mammalian cells. Because eosinophilia is characteristic of asthma, we tested the effect of MBP on bronchi and assayed sputa for this protein. We found that MBP damaged bronchial epithelium in vitro and produced changes that mimicked those in asthma. Radioimmunoassay of sputa from 100 consecutive patients with respiratory diseases revealed MBP levels above 0.1 mug/ml in 13 patients, and 11 of these had asthma. In 15 patient hospitalized for asthma, MBP levels of sputum were markedly elevated. Treatment with bronchodilators and glucocorticoids caused an increase peak expiratory flow rate, a reduction in blood eosinophils, and a decrease in the serum and sputum levels of MBP. The results indicate that eosinophil granule constituents are released into the bronchi in asthma and that measurement of sputum MBP may be useful in identifying asthma. The possibility that the eosinophil damages bronchial epithelium in asthma is discussed.