We developed a post-embedding ultrastructural enzyme affinity-gold technique to label histamine. Diamine oxidase (DAO)-gold complex was prepared and tested for specificity with routinely processed, Epon-embedded, histamine-rich human mast-cell granules and histamine-agar test blocks, both of which were labeled. Specificity controls included removal of staining by DAO digestion of Epon sections containing mast cells or by filtering DAO-gold over histamine-agarose beads, and a wide variety of controls (for these specificity controls) that did not abrogate mast-cell granule or histamine-agar block labeling with DAO-gold. Internal negative controls in each sample included the failure of DAO-gold to stain Type II alveolar pneumocyte lamellar bodies or altered, swollen mast-cell granules from which histamine has been released. The new post-embedding enzyme affinity-gold technique is effective in optimally prepared ultrastructural samples that allow clear identification of key ultrastructural morphology.