Ferritin was isolated from the livers and brains of two groups of rats, one of which was fed aluminum chloride (100 microM) for 1 year in the drinking water. Brain tissue contained about one-third of the amount of ferritin found in the liver. While brain ferritin from normal rats contained 42.1 +/- 14.3 mol of aluminum, that from the aluminum-fed group contained 115.4 +/- 48.3 mol of aluminum per mol of ferritin. Liver ferritin from both groups contained similar amounts of both aluminum and iron, and the amounts were less than that found associated with brain ferritin. Ferritin isolated from the brains of patients who died of Alzheimer disease contained more aluminum and more iron than that from age-matched controls. Human brain ferritin is composed of two types of subunits--about 70% heavy chain (Mr, 22,000) and 30% light chain (Mr, 19,500). The isoelectric focusing pattern of human brain ferritin was considerably different from that of human liver. Only 5 of the 20 brain ferritin bands migrated similarly to the acidic isoferritins from the liver, and the major component of brain ferritin, representing 30% of the total ferritin, had a pI of 8.0.