Olopatadine hydrochloride (olopatadine; KW-4679), (Z)-11-[(3-dimethylamino)propylidene]-6,11-dihydrodibenz[b,e]oxepin-2-acetic acid monohydrochloride, is an antiallergic drug with selective and potent histamine H1 receptor antagonist activity. In the present study, we investigated the effect of olopatadine on the release of peptide leukotrienes (P-LT), potent inflammatory mediators, from human eosinophils. Human eosinophils were purified from venous blood of healthy donors by negative selection using the anti-CD16 antibody. When human eosinophils were stimu- lated with the calcium ionophore A23187 (1 µmol/L), the amount of P-LT release was approximately 1200 pg/ 105 cells, while thromboxane (TX) B2 release was under the detection limit (< 20 pg/105 cells). Olopatadine inhibited the A23187-induced P-LT release from human eosinophils with an IC50 of 4.5 µmol/L. Ketotifen also inhibited this reaction with an IC50 of 39.4 µmol/L. The inhibitory effect of olopatadine on the P-LT release may contribute to the antiallergic efficacy of this drug.