1. Mast cells make up between 0.5 and 3% (mean 1.35%) of total cells recovered by bronchoalveolar lavage (BAL). 2. The majority of these cells have the morphological characteristics of mucosal mast cells in that they fail to stain in the alcian blue-safranin reaction after fixation in formol-saline but stain well after fixation in Carnoy's solution. Cells staining with berberine sulphate were seen in only four of the 26 lavages. 3. BAL cells released histamine in response to anti-human immunoglobulin E (IgE) in a dose-dependent manner that was optimal at a dilution of anti-IgE of 1:100. Maximum release was obtained by 2 min. 4. Histamine release was completely inhibited by a combination of 2-deoxyglucose (5 mmol/l) and antimycin A (1 μmol/l). 5. Disodium cromoglycate (DSCG) significantly inhibited this histamine release at 1 mmol/l ( P <0.02), 100 μmol/l ( P <0.002) and 10 μmol/l ( P <0.003), with maximum inhibition of 50.1% at 10 μmol/l.