When hesperidin isolated from pericarpium of Citrus unshiu (family Rutaceae) was incubated with human intestinal microflora, its main metabolite was hesperetin, which was a main metabolite in urine of orally hesperidin-administered rats. The antiallergic activity of hesperidin and its metabolite hesperetin were investigated. Hesperidin did not inhibit the histamine release from RBL-2H3 cells induced by IgE. However, its metabolite hesperetin potently inhibited the histamine release from RBL-2H3 cells induced by IgE and the PCA reaction. The inhibitory activity of hesperetin was found to be comparable with azelastine, a commercially available antiallergic drug, and to potently inhibit prostaglandin E2 production in lipopolysaccharide-stimulated RAW 264.7 cells. Hesperetin weakly inhibits cyclooxygenase 2 enzyme activities. These results suggest that hesperidin may be a prodrug, which is metabolized to hesperetin by intestinal bacteria.