Biopsy specimens of human ilea were prepared for the ultrastructural detection of histamine in vivo with a new enzymeaffinity method (Dvorak et al., J Histochem Cytochem 1993; 41:787–800). Human gut mucosal mast cells contained histamine in cytoplasmic secretory granules that were electron dense, regardless of underlying substructural patterns. Partially and completely empty cytoplasmic granules, characteristic for piecemeal degranulation (Dvorak et al., Int Arch Allergy Immunol 1992;99:74–83) had diminished and absent histamine stores, whereas electron-dense granules in the same cells retained their histamine. interstitial collagen (but not basal lamina of other cells) near secretory mast cells avidly bound histamine, thereby providing a potential source for re-release of extracellular histamine stores. These studies provide the first documentation of histamine secretion in vivo during piecemeal degranulation of human mast cells.