A relationship of cellular composition in the airways and the release of chemical mediators from the cells to bronchial hyperresponsiveness was examined in two age-matched asthma groups: 15 atopic and 15 nonatopic patients. 1. A significant correlation between the proportion of eosinophils in bronchoalveolar lavage (BAL) fluid and bronchial reactivity (BR) was found in patients with atopic and nonatopic asthma. The proportion of the cells (combined eosinophils and neutrophils) in BAL fluid was closely correlated with BR in patients with atopic asthma, but not in those with nonatopic asthma. There was no correlation between the proportion of BAL neutrophils and BR in the two asthma groups. 2. There was a significant correlation between histamine release from BAL cells and BR in patients with atopic asthma. In contrast, LTC4 release from BAL cells was significantly corrected with BR in patients with nonatopic asthma. The results suggest that the humoral and cellular components in the airways, that participate in bronchial hyperresponsiveness, are different between patients with atopic and nonatopic asthma.