AbstractThis unit describes methods for measuring exocytosis of preformed mediators from secretory granules as an indication of IgE receptor‐mediated activation of mast cells. The first basic protocol describes the measurement of biogenic amines (serotonin and histamine) secreted by activated rodent mast cells (for serotonin) or rodent and human mast cells (for histamine). The second basic and alternate protocols detail techniques for measuring the release of β‐glucuronidase, an enzyme that is synthesized by human and rodent mast cells, stored in secretory granules, and released during degranulation. Methods for assaying other enzymes released during degranulation, such as β‐hexosaminidase and tryptase, are discussed in the . These protocols can also be applied to basophils where appropriate.