Cetirizine, a metabolite of hydroxyzine, is an antihistamine with as distinguishing features: 1) exquisite anti-H1 specificity: cetirizine appears unique in being devoid of action on receptors other than the H1 receptor; 2) potency: at unit dose it is the most potent antihistamine in the skin and the lung; 3) absence of metabolism. These three characteristics suggest that cetirizine be considered the choice H1 antagonist for experiments on the immediate allergic reaction. Cetirizine additionally in vitro inhibits the migration of eosinophils, and in vivo, in the skin, the infiltration by eosinophils that is characteristic for the late phase allergic reaction. Other antihistamines are less active or inactive with respect to this property. According to several lines of evidence, the effect of cetirizine on eosinophils is unlikely to be due to H1 antagonism, but is more likely a novel property of the compound.