The effects of activated platelets on the isolated airway hyperreactivity induced by antigen or CCh were investigated. After the activated platelets were incubated with the sensitized airway for about 5 min, the reactivity of the airway to OA or CCh increased; when the platelets were depleted over 90%, it inhibited the reactivity of the airway to OA or CCh, suggesting that platelets might play an important role in the ariway hyperresponsiveness in guinea pigs, and affect the hyperreactivity of the large and small airways.