Concentrations of main bronchoconstricting chemical mediators, histamine and leukotriene C4 (LTC4), were measured in bronchoalveolar lavage (BAL) fluid, and when cells (peripheral leukocytes and BAL cells) were stimulated by Ca ionophore A23187, in 7 atopic and 7 nonatopic asthma patients. 1. The proportion of basophilic cells was significantly larger in atopic than in nonatopic asthma (p<0.05), however no significant difference was present in the other BAL cells between the two asthma types. 2. Concentration of histamine in BAL fluid was significantly higher in atopic than that in nonatopic asthma, however, difference in that of LTC4 was not found between them. 4. The release of LTC 4 from BAL cells was higher in nonatopic than that in atopic asthma, but this was not significant. In contrast, the release of histamine was significantly higher in atopic compared to that in nonatopic asthma (p<0.001) when the cells were stimulated by Ca ionophore A23187. These results suggest that both histamine and LTC4 participate in the onset mechanism of atopic asthma, and only LTC4 in that of nonatopic asthma.