Ferritin has been isolated and its subunit composition, iron and aluminium content determined in the cerebral cortex and cerebellum of normal individuals and in the cerebral cortex of Alzheimer's-disease and renal-dialysis patients. An e.l.i.s.a. for ferritin has been developed and the ferritin, non-haem iron and aluminium content of the parietal cortex were determined in normal individuals and Alzheimer's-disease patients. It was found that ferritin from the cerebral cortex and cerebellum of normal individuals had a high H-subunit content, similar to that of heart ferritin. The subunit composition of ferritin isolated from the cerebral cortex was not significantly altered in Alzheimer's-disease or renal-dialysis patients. Ferritin from the cerebral cortex of normal individuals had only approx. 1500 atoms of iron per molecule and the iron content of ferritin was not significantly changed in Alzheimer's-disease or renal-dialysis patients. Ferritin isolated from the cerebral cortex of normal, Alzheimer's-disease and renal-dialysis patients had less than 9 atoms of aluminium per molecule. The failure to find increased concentrations of aluminium associated with ferritin in dialysis patients, who had markedly increased concentrations of aluminium in the cerebral cortex, shows that aluminium does not accumulate in ferritin in vivo. This has important implications for the toxicity of aluminium, since it implies that cells are unable to detoxify aluminium by the same mechanism as that available for iron. Comparison of the concentrations of ferritin, aluminium and iron in the parietal cortex from normal and Alzheimer's-disease patients showed that, whereas the concentration of aluminium was not increased, both ferritin and iron were significantly increased in Alzheimer's disease.