Protease-activated receptors (PARs) are a unique class of G protein-coupled receptors, which are activated by proteolytic cleavage of the amino terminus of the receptor itself. PARS are most likely involved in various biological responses, such as hemostasis and regulation of muscle tone; however, the roles of PARS in the functions of inflammatory and immune cells are poorly understood. Because eosinophils are most likely involved in allergic inflammation and are exposed to a variety of proteases derived from allergens and other inflammatory cells, we investigated whether PARS regulate effector functions of eosinophils. Human eosinophils constitutively transcribe mRNA for PAR2 and PAR3, but not those for PAR1 and PAR4. The expression of PAR2 protein was confirmed by flow cytometry. When trypsin, an agonist for PAR2, was incubated with eosinophils, it potently induced superoxide anion production and degranulation; 5 nM trypsin induced responses that were 50 similar to 70% of those induced by 100 nM platelet-activating factor, a positive control. In contrast, thrombin, an activator for PARI, PAR3, and PAR4, showed minimal effects. The stimulatory effect of trypsin was dependent on its serine protease activity and was blocked 59% by anti-PAR2 Ab. Furthermore, a specific tethered peptide ligand for PAR2 potently induced superoxide production and degranulation; the effects of peptide ligands for PAR1, PAR3, and PAR4 were negligible. These findings suggest that human eosinophils express functional PAR2, and serine proteases at the inflammation site may play important roles in regulating effector functions of human eosinophils. The expression and functional relevance of other PARS still need to be determined.