We examined the staining characteristics and degranulation of mast cells in bronchial biopsy specimens taken by fiberoptic bronchoscopy from 13 stable asthmatic patients and eight normal nonsmoking subjects. Specimens were fixed in periodate-lysine-paraformaldehyde, embedded in glycol methacrylate, and stained with toluidine blue (2%) for 30 min (pH 2.7) and 7 days (pH 0.5). The number of mast cells in the epithelium and in the lamina propria was counted under light microscopy. In addition, the distribution of mast cells with different granule contents, arbitrarily defined as degranulated or partly degranulated and fully granulated, was estimated at the two levels. In asthmatic subjects, the number of mast cells in the epithelium after either staining method was significantly higher compared with that in control subjects. The number of mast cells in the lamina propria, but not in the epithelium, was significantly higher after 7 days compared with 30-min toluidine blue stain both in asthmatic (135.6/mm2 versus 74.8/mm2; p < 0.001) and control subjects (121.5/mm2 versus 71.5/mm2; p < 0.01). There was evidence of a progressive mast cell degranulation when moving toward the airway lumen in both groups. However, degranulation was more evident in asthmatic subjects. In both groups, granulated mast cells were absent in the epithelium, whereas in the lamina propria granulated mast cells were approximately one-third of total in asthmatic and two-thirds of total in normal subjects. These observations suggest that mast cells in human bronchial mucosa are heterogeneous with respect to histochemical characteristics. They provide evidence that degranulation of mast cells occurs in both asthmatic and normal subjects and that degranulation is greater in asthmatics.