RWJ-58643 is a β-trypsin inhibitor that suppresses allergic inflammation. RWJ-58643 (100 mg) significantly reduced nasal allergy symptoms, eosinophil infiltration, and IL-5 levels at low doses. RWJ-58643 resulted in late eosinophilia and elevated IL-5 levels at high doses (300 mg and 600 mg). RWJ-58643 modulates allergic responses by inhibiting the release of β-trypsin from mast cells.