The antiallergic drug, cetirizine, inhibits the histamine release from a rat basophilic leukemia (RBL-2H3) cell line, which is frequently used as a mast cell model. By investigating inhibitory activities of (+)- and (-)-cetirizine in RBL-2H3 cells on the histamine release, we aimed to evaluate the effect of their structual characteristics on the antihistamine activity. The study on RBL-2H3 cell has clearly demonstrated that the (-)-cetirizine is significantly more potent than the (+)- or the racemic cetirizine, although there was no difference in pharmacokinetics between (+)- and (-)-cetirizine in rats.