The preventive effect of ketotifen, a new drug with anti-histaminic and antiallergic properties, on histamine-induced bronchoconstriction was studied by open assessment in twenty-four adult patients with extrinsic asthma. A single oral dose of 1 mg ketotifen reduced the post-histamine mean drop in peak expiratory flow from 33 to 16% of the basal values (P<0.001). After a 4 weeks' regimen of 1 mg ketotifen twice daily the post-drug histamine-induced fall in PEF was further significantly reduced (P<0.001). Tests performed after 8 and 12 weeks of treatment showed no additional decrease in bronchial reactivity to histamine. Tests performed 1 week after cessation of treatment showed return of bronchial reactivity to the pretreatment level. The results suggest that a single dose of ketotifen has a marked preventive effect on histamine-induced bronchoconstriction and that this effect is enhanced during continued treatment.