Ultrastructural localization of human thyroid peroxidase in normal and hyperfunctioning states has been studied. Peroxidase activity was visualized ultrastructurally with a cytochemical reaction using 0.05% diaminobenzidine-tetrahydrochloride, and hydrogen peroxide with a final concentration of 0.0025%, 0.005%, or 0.01%, respectively, and 2% osmium tetroxide. Reaction product demonstrating the enzyme was clearly observed in cell organelles with a final concentration of hydrogen peroxide at 0.0025% or 0.005%. In the normal portion of thyroid, follicular cells demonstrated the reaction product, located mainly in rough-surfaced endoplasmic reticulum and perinuclear cisternae and in addition a small amount of the product was found in Golgi cisternae and small vesicles dispersed throughout the subapical cytoplasm. The reaction product significantly increased in amount in the cell organelles described above and in addition, a characteristic heavy deposition of the product was observed at the external surface of the microvilli in the follicular cells of thyroid obtained from the patients with treated Basedow's disease.