The effects of cetirizine (C), a new generation, nonsedative, H 1 -antihistamine drug, were studied on human isolated bronchi. C is a potent antagonist of the bronchial muscle contraction induced by histamine, irrespective of whether C is administered by cumulative addition or prophylactically. In the former case, this effect of C was significant at a concentration of 3 × 10 −8 mol/L and was maximal at a concentration of 10 −5 mol/L. The −log of concentration of C causing 50% of the maximal effect induced was 7.30 ± 0.05 (n = 6), and the effect produced was not significantly modified by bronchial epithelium removal. When C was administered prophylactically, the concentration-response curves to histamine were displaced to the right, but the reduction of maximum histamine response suggests a noncompetitive type of antagonism. C is devoid of notable anticholinergic effects. At concentrations of 10 −8 to 10 −7 mol/L, C proved capable of enhancing the relaxant effect produced by salbutamol (10 −7 to 3 × 10 −7 mol/L) on human isolated bronchi that had been contracted by either histamine or acetylcholine (ACh). The synergy appeared to be additive or potentiating, depending on salbutamol (SAL) concentrations. Under similar conditions, mepyramine does not potentiate the effects of SAL against histamine. Finally, at concentrations of 10 −8 to 10 −6 mol/L, C reduced the functional antagonism observed between SAL and ACh, as can be observed by the increase, in the presence of C, of the maximal relaxant effect of SAL on contractions produced by 10 −3 mol/L of ACh. We may, therefore, conclude that C appears to be a specific antihistamine on human isolated bronchi and that it appears to potentiate the bronchodilator effect of SAL on this preparation.