Mucosal mast cells in human nasal turbinates obtained at surgery were studied by electron microscopy before and after exposure en bloc to a degranulating stimulus. Mast cells occurred as solitary cells and in islets typically containing one mast cell and one to three mononuclear cells. All islets and most solitary mast cells were associated with thin cytoplasmic processes of distinctive, branched stromal cells that also phagocytosed debris from pyknotic mast cells. Mast cells were grouped into three categories based largely on published ultrastructural criteria. Resting (nondegranulating) mast cells possessed secretory granules that typically packed the cytoplasm and contained a densely stained amorphous material and scroll-like crystalline profiles. Granules in some resting cells were larger, more polymorphic, and appeared nearly homogeneous. Secretory granules of degranulating mast cells possessed crystalline, scroll-like, or reticular constituents predominantly with interspersed amorphous material. Granules were fewer in number and were often concentrated in the peripheral cytoplasm. Few degranulating mast cells exhibited labyrinth formation or exocytosis of granule contents. Largely degranulated mast cells had few if any typical granules but possessed small vacuoles containing recognizable granule remnants. Resting and degranulating mast cells possessed very long cell surface projections that, in places, interdigitated to form stacks of parallel folds above the plasmalemma, resulting in a threefold increase in cell-surface area. The surface membrane of the folds appeared to be continuous with that lining intracytoplasmic channels extending from the cell surface to the Golgi zone.