Background: Mast cells and eosinophils are important effector cells in asthma. Understanding their interactions is essential for studying asthma pathophysiology. Inflammatory mediators released from mast cells, such as arachidonic acid metabolites, TNF and IL-5, are important in eosinophil biology. However, little is known about the effects of mast cell-specific mediators, such as tryptase, on eosinophils. Our objective was to investigate the effects of mast cell tryptase on human peripheral blood eosinophils. Methods: Peripheral blood eosinophils isolated from asthmatic individuals were activated using various concentrations of tryptase- and protease-activated receptor-2 (PAR-2)-activating peptides (PAR-2 AP). Eosinophil activation was evaluated by the release of granule mediators, superoxide release, estimation of eosinophil survival, changes in intracellular Ca²⁺ concentration and mitogen-activated protein kinase activation. Results: Tryptase induced the release of eosinophil peroxidase and β-hexosaminidase from peripheral blood eosinophils but had no effect on RANTES release. Eosinophils isolated from two thirds of our donors responded to tryptase, while the remainder appeared not to respond. Release of granule mediators was dependent on tryptase enzymatic activity. To identify the mechanism of eosinophil activation by tryptase, we studied the expression of PAR-2 by eosinophils and its function. Using RT-PCR, we amplified PAR-2 from eosinophils. However, flow cytometry failed to detect significant PAR-2 expression on the surface of eosinophils. The PAR-2 AP SLIGRL-NH₂ did not induce eosinophil activation by any of the methods we employed. Conclusion: Our data indicate that mast cell tryptase may affect eosinophil activation status independently of PAR-2.