Bronchoalveolar lavage was performed in 97 patients including control patients with bronchial carcinoma (24) and patients with sarcoidosis (20), cryptogenic fibrosing alveolitis (9), and asthma (4), and others. Cytocentrifuged slides were stained by two methods: May-Grünwald Giemsa and toluidine blue. In the last 32 subjects the bronchoalveolar lavage fluid was separated into supernatant and cell pellet for the subsequent assay of the performed mast cell mediator, histamine. Comparison of the two methods of staining showed a bias towards toluidine blue. Controls had a differential mean (SE) mast cell count of 0.07% (0.01%). Higher counts were noted in cryptogenic fibrosing alveolitis--0.61% (0.15%) (p less than 0.001)--and in sarcoidosis--0.14% (0.02%) (p less than 0.05). There was a strong correlation between absolute mast cell counts and cell lysate histamine concentration (r = 0.78, p less than 0.001). Less strong, significant, correlations between supernatant histamine concentration and absolute mast cell counts (r = 0.48, p less than 0.01) or cell lysate histamine concentration (r = 0.72, p less than 0.01) were also found. Derived mean values of histamine per mast cell ranged from 3.7 to 10.9 picograms. The mean histamine content of lavage fluid supernatant as a percentage of the total lavage fluid histamine was 24.9% (3.3%). The possible clinical significance of these findings is discussed.