One of the characteristics of patients with bronchial asthma is activation of eosinophils in the bronchi, which can be evaluated by measuring concentrations of the cationic granule proteins. Sputum seems to be the best material to evaluate the activation of eosinophils in the bronchi because it can be collected easily from the same patient every day even if he (she) has an asthmatic attack. Recently it has become possible to measure the concentration of the eosinophil cationic protein (ECP) thanks to the availability of the ECP RIA kit. However no fundamental studies have been carried out on the handling of sputum samples, and whether only ECP released into sputum is measured. We prepared supernatants of sputum samples according to the method of Gleich and others by adding the same volume of physiological saline, mixing it for one minute by a vortex mixer and centrifuging at 40000 Xg, 4 C, for 30 minutes. We found that the measurement of ECP is not inhibited by materials in sputum supernatants through a dilution test and a recovery test. We also found that the followings did not influence the measurement of ECP; the times of dilution, the duration from collection to handing of sputum, the solution added to sputum, and the gravity of centrifugation. In addition, it was suggested that only ECP released into sputum is measurable from the results of an electron microscopic study and the measurement of ECP after centrifugation following the addition of white blood cells to sputum.