A new enzyme-affinity-gold ultrastructural method makes use of the affinity of the enzyme, diamine oxidase coupled to gold, for its substrate, histamine, for localization of histamine in isolated human lung mast cells (HLMCs). The method works with routinely prepared ultrastructural samples, thereby allowing precise identification of ultrastructural structures that contain histamine. We used this method to identify the release of histamine from granule stores in anti-immunoglobulin-E (IgE)-stimulated HLMCs and the replacement of histamine in secretory granules of HLMCs during recovery from anaphylactic degranulation in vitro. The findings show that electron-dense granules in unstimulated HLMCs, maintained up to 24 h in culture, and in responding anti-IgE-stimulated HLMCs, over the same time period in vitro, contained histamine. Alteration of granules, resulting in decreased electron density of their contents, was associated with decreased label for histamine. Electron-lucent intracytoplasmic degranulation chambers were devoid of histamine. Recovering HLMCs developed new granule stores of histamine by a mixture of conservation, synthetic, and endocytotic mechanisms.