Mast cells have been implicated in the pathogenesis of allergic asthma but their role in non-allergic asthma remains to be elucidated. The spontaneous and non-specific release of histamine by suboptimal doses of calcium ionophore A23187 was studied in bronchoalveolar lavage cells obtained from nine asthmatic and seven healthy individuals. Bronchoalveolar lavage was performed with saline, and total cells were incubated without any secretagogue (spontaneous histamine release) or after addition of 1.25, 2.5 and 5 μm of A23187 for 30 min (net maximal release). Histamine was titrated by using a very sensitive radioimmunoassay using a monoclonal antibody against acylated histamine. The spontaneous release was similar in asthmatic (20.6±8.2%) and healthy individuals (17.4±8.4%). The net maximal release of histamine was significantly greater in asthmatic patients (28.1±17.4%) than in normal subjects (10.3±8.9%). The release of histamine was significantly correlated to the release of PGD2 measured by enzyme immunoassay using a polyclonal antibody against methoxamine-PGD2 (Spearman rank test: 0.78, P<0.01). In eight subjects, the release of histamine by A23187 was studied in the presence of nedocromil sodium and it was observed that this drug significantly (P<0.05) decreased the net maximal release of histamine. This study shows that mast cells from asthmatic individuals have a greater releasability than those of normal subjects.