The mast cell is a multipotent inflammatory cell that has been shown to participate in the pathogenesis of a variety of diseases, such as immediate hypersensitivity reactions, arthritis, atherosclerosis, and heart failure. Upon stimulation, mast cells exocytose cytoplasmic secretory granules into their extracellular microenvironment. These granules are modified lysosomes containing preformed mediators such as histamine, neutral proteases, cytokines, and growth factors embedded in a heparin proteoglycan matrix. When exposed to the extracellular fluid, the soluble components of the granules (e.g., histamine and cytokines) diffuse away, whereas the heparin proteoglycans and the mast cell-specific neutral proteases (e.g., chymase) remain tightly bound to each other, forming proteolytically active intra- and extracellular granule remnants. This unit describes a method to isolate rat serosa mast cell granule remnants in experiments aimed at determining the role of mast cell activation and degranulation in disease.