Fixation and staining characteristics were studied for mast cells recovered by bronchoalveolar lavage from 67 patients being investigated for lung disease. The number of toluidine blue stained mast cells in formaldehyde-fixed cytocentrifuge preparations was consistently less than in specimens fixed in Carnoy's solution, though the counts were highly dependent on the period of fixation or staining. The cellular histamine content closely correlated with total mast cell numbers in bronchoalveolar lavage fluid, but was not related to the relative proportions of mast cells which were sensitive or resistant to formaldehyde fixation when using a standard protocol. Compared with normal subjects, the numbers of formaldehyde-sensitive mast cells were significantly elevated in patients with bronchial carcinoma, sarcoidosis, extrinsic allergic alveolitis, cryptogenic fibrosing alveolitis, and mycobacterial infection and were particularly high in the cases of interstitial lung disease. An even greater increase in numbers of formaldehyde-resistant mast cells was observed in the patients with sarcoidosis and extrinsic allergic alveolitis. The associations of these mast cell subsets with disease may reflect relationships between the expansion of the formaldehyde-sensitive population and lymphocyte infiltration and between proliferation of formaldehyde-resistant mast cells and tissue fibrosis.