In advanced cases of superficial siderosis of the human central nervous system, the clinical triad of hearing loss, cerebellar ataxia, and myelopathy permits the diagnosis at the bedside, and magnetic resonance imaging readily confirms the hemosiderin deposits in brainstem, cerebellum, and spinal cord. To study the pathogenesis of this condition and explain the selective vulnerability of the cerebellum, experimental siderosis was induced in rabbits by the repeated intracisternal injection of autologous red blood cells. The earliest cellular response in the cerebellar molecular layer was hyperplasia and hypertrophy of microglia as displayed by immunocytochemistry for ferritin. Microglia also contained iron, but ferritin biosynthesis appeared to proceed without commensurate iron accumulation. This early apoferritin response probably occurred due to the presence of heme, rather than iron, in the cerebrospinal fluid and subpial tissue. Ferritin biosynthesis is accelerated when the ferritin repressor protein is dissociated from ferritin messenger ribonucleic acid. A specific antiserum localized ferritin repressor protein predominantly to astrocytes including Bergmann glia. It is proposed that abundance and proximity of ferritin repressor protein--immunoreactive Bergmann glia and ferritin-containing microglia in the cerebellar molecular layer permit prompt cellular interaction in the conversion of heme to ferritin and ultimately hemosiderin.