The hypothesis studied is that increased responsiveness in asthma is not limited to the airways. Forty asthmatic children were analysed for their bronchial responsiveness (BR) to exercise. Twenty patients revealed bronchial obstruction after exercise while the remainder did not. These observations were compared with the responsiveness of leucocytes, which was determined by their histamine ‘releasability’. Twenty healthy children served as controls. Release of histamine induced by calcium ionophore-aided calcium influx was significantly higher in both groups of asthmatics than in the healthy children (P < 0.005). Similar findings were obtained by induction of microtubule aggregation due to deuterium oxide (D2O). The S-shaped dose-response relationship with D2O was shifted to the left in the patients with BR to exercise compared to patients without (P < 0.025). The slope was increased in both patient groups compared with the healthy children (P < 0.01). It is concluded that the mean ‘releasability’ of histamine release due to both stimulants correlated well (P < 0.01). This suggests that the ‘releasability’ is determined by the responsiveness of the microtubules. This may also apply to allergen-induced histamine release, as was revealed from studies with anti-IgE. The differences in histamine release found in relation to BR due to exercise were also present if the patients were divided according to BR due to histamine. A significant relationship existed between the degree of BR to histamine and the responsiveness of the microtubules (P < 0.02).