We have studied various functional and morphological characteristics of mast cells obtained in bronchoalveolar lavage from fifty-two patients with several lung diseases. The percentage of mast cells ranged from 0.04 to 0.6% (bronchial carcinoma). 0.05–0.3% (sarcoidosis), 0.06–0.25% (asthma), 0.04–1.8% (miscellaneous) and 0.02–0.04% (normals). There were no significant differences in the mast cell counts between the disease groups. Lung mast cells exhibited heterogeneity of size, shape and intensity of staining. Cells from thirty-seven subjects were further studied for total histamine content and histamine release using various secretagogues. There was a significant correlation (P<0.001) between the histamine content of the total lavage cell population and mast cell counts. The calculated mean histamine content per mast cell was 6.35 pg. Histamine was released in a dose-dependent fashion after stimulation with anti-IgE, calcium ionophore and phorbol myristate acetate with a time course of histamine release characteristic of the mast cell. Unlike peripheral blood basophils, no release was observed following incubation with f-met-leu-phe (10−6-10−8m) and neither cell type released histamine following incubation with 48/80 (10 μg/ml). Inhibition of anti-IgE-induced histamine release was obtained following pre-incubation with salbutamol (10−4-10−6m). These studies indicate that bronchoalveolar lavage is a suitable model for the study of human lung mast cells.