Brain tissue from five patients with superficial siderosis of the central nervous system was examined by immunocytochemistry for ferritin, glial fibrillary acidic protein (GFAP), alpha 1-antitrypsin, and alpha 1-antichymotrypsin, and by lectin affinity cytochemistry with biotinylated Ricinus communis agglutinin-1 (RCA-1). The sections were pretreated with 2,2'-dipyridyl and sodium hydrosulfite to remove iron and to reveal the antigenic sites. In siderotic cerebellar cortex, ferritin reaction product occurred in the hemosiderin matrix, the cell bodies and processes of Bergmann glia, and in microglia. Astrocytes other than Bergmann glia did not contain ferritin reaction product. RCA-1 stained microglia and hemosiderin whereas antisera to alpha 1-antitrypsin and alpha 1-antichymotrypsin only reacted with iron-depleted granules. The selective vulnerability of the eighth cranial nerve was explained by the presence of ferritin-reactive and lectin-positive microglia. Hemosiderin isolated from frozen cerebellum contained ferritin, GFAP, and vimentin. The presence of the intermediate filament proteins was likely due to co-localization with hemosiderin granules in Bergmann glia. The ability of the brain to biosynthesize ferritin in response to prolonged contact with hemoglobin iron is thought to be the most important factor in the pathogenesis of superficial siderosis. The great severity of the lesion in the exposed cerebellar cortex is readily explained by accelerated ferritin biosynthesis in Bergmann glia.