BACKGROUND: Eosinophilic leukocytes are prominent cellular participants in the pathogenesis of allergic disease and asthma. Chemotaxis is still a very useful method in evaluating the response of human eosinophil to novel modulators. Degranulated mast cells and activated T lymphocytes are responsible for the pathophysiology of asthma and tryptase is one of most important proteases released after activation of mast cells. The purpose of this study was to investigate the actions of trypsin and chymotrypsin on eosinophils in terms of chemotaxis and activation. METHOD: Eosinophils were isolated by negative immunoselection from the peripheral blood of atopic donors. Chemotaxis was studied by using micro-Boyden chambers and ECP release was assayed by fluoroimmunoassay. RESULTS: Eosinophil showed a chemotactic response to trypsin. Maximal chemotactic response was with 1000microg/ml trypsin (56.52 +/- 14.50/HPF) which was comparable to PAF. But chymotrypsin showed no significant chemotactic response to eosinophils. Trypsin at the concentration of 10, 100,1000microg/ml induced secretion of ECP, which at the concentration of 10microg/ml represented about 2.7 times of the spontaneous rate of release. Soybean protease inhibitor reduced trypsin induced ECP release. CONCLUSION: Trypsin can induce chemotactic response to eosinophils and activation of eosinophils that can induce secretion of ECP. On the contrary, chymotrypsin showed no direct effect on eosinophils. We propose a role of trypsin on the chemotaxis and activation of eosinophils.