Sputum eosinophilia is characteristic of bronchial asthma, and sputum specimens from patients with asthma contain eosinophil-derived substances including Charcot-Leyden crystals (CLC) and the eosinophil granule major basic protein (MBP). Prior observations have indicated that an elevated sputum level of MBP is associated with asthma. To determine whether CLC protein was also elevated in asthma, we measured by specific radioimmunoassays the quantities of MBP and CLC protein in sputum specimens from 106 consecutive patients with various respiratory diseases and in sputum specimens from 10 patients hospitalized for asthma. The CLC protein was detected in all sputum samples, and the levels in the 116 samples averaged 3.5 micrograms/ml (range, 0.01 to 25 micrograms/ml). The CLC protein was significantly elevated in sputum specimens from patients with acute asthma and from patients with certain other respiratory diseases when associated with bronchopulmonary infection. In contrast, MBP levels in the 116 samples averaged 0.32 micrograms/ml (range, 0.01 to 8.8 micrograms/ml) and were significantly elevated only in patients with asymptomatic or acute asthma. Sputum MBP was not elevated in patients with bronchopulmonary infections unless acute asthma was also present. Thus, an elevated sputum MBP level was specifically associated with asthma, whereas an elevated CLC protein level was associated with both asthma and with bronchopulmonary infection in certain patient groups.