Cetirizine, a potent Hrantagonist, has been reported to inhibit eosinophil migration into human skin. We, therefore, further evaluated the effect of cetirizine on eosinophil function, including superoxide anion generation, chemotaxis, and eosinophil peroxidase (EP) release. In allergic subjects, superoxide anion generation 60 min after platelet-activating factor (PAF) activation was inhibited by concentrations of cetirizine ranging from 0.01 to 1 μg/ml (2.612 × 10^––8 to 2.612 × 10^––6M). No significant inhibition was observed in normal subjects. PAF (10^––6M)-induced eosinophil chemotaxis was also inhibited by cetirizine. In allergic subjects, percent inhibitions were 47.5 ± 6.1% at 0.01 μg/ml, 50.8 ± 5.1% at 0.1 μg/ml and 58.9 ± 6.4% at 1 μg/ml of cetirizine. In allergic subjects, N-formyl-methionyl-lencyl-phenylalanine induced eosinophil chemotaxis was inhibited by cetirizine, although EP release was not. These results suggest cetirizine has effects on eosinophils which can not be explained by H₁-blockade alone.