The mechanism underlying airway hyperresponsiveness in asthma is unknown although an abnormality in the airway smooth muscle resulting in decreased relaxation or increased contractile response has been proposed. The present study was designed to demonstrate any differences in the in vitro sensitivity of airway smooth muscle between asthmatic patients and non-asthmatic patients. Using bronchial tissue obtained by resection from mild to moderate asthmatic patients and from non-asthmatic patients, we have shown that the altered airway responsiveness seen in asthmatic patients is not reflected in airway smooth muscle sensitivity in vitro. Sensitivity of the bronchial smooth muscle to isoprenaline and aminophylline or theophylline did not differ between asthmatic patients and nonasthmatic patients, while sensitivity to carbachol and histamine was significantly reduced in tissue from asthmatic patients. These results suggest that the abnormality in asthma may not lie at the level of the airway smooth muscle.