Structural heterogeneity of mast cells in human bronchial mucosa was investigated by examining 100 cells by electron microscopy and morphometry. Differential counts of secretory granules allowed subdivision of mast cells into three groups: (1) 49 cells with greater than 65% mixed granules; (2) 30 cells with greater than 30% scrolled granules; (3) 12 cells with greater than 30% particulate granules. Nine cells showed borderline characteristics. Records of depths of cells in the mucosa showed that most mixed-granule cells occupied middle levels, whereas most scrolled granule cells lay higher up, near the basement membrane. This raised the possibility that scrolled-granule cells may represent partly degranulated mixed-granule cells. Nineteen mast cells were filled with very dense mixed granules, and appeared to correspond to those staining with safranin in paraffin sections. However, morphometry did not produce any criteria for distinguishing these cells sharply from paler mixed-granule cells, which were therefore regarded as degranulating forms of the same type of cell. Evidence was found of a gradient of mast cell degranulation which appeared to increase in magnitude upward from deep submucosa to superficial mucosa. This evidence included finding a significant upward reduction in total granule area per cell, total granule numbers, numbers of mixed granules, and numbers of dense-cored granules. It was concluded that although bronchial mucosal mast cells could be subdivided ultrastructurally into three apparently heterogeneous groups, degranulation was found to produce a wide range of different cell appearances, and could, conceivably, even be responsible for the above grouping, rather than intrinsic mast cell heterogeneity.