Immunocytochemical studies on the adult human brain have shown that transferrin is localized within three main compartments in the adult human brain. Oligodendrocytes and some astrocytes together with cells of the choroid plexus showed the highest intensity of staining. Neuronal staining occurred mainly within pyramidal or large polygonal cells, but this showed considerable regional variation being most marked in areas such as the cerebral cortex, amygdala, hippocampus, brainstem and cerebellar Purkinje cells. Small neurones such as caudate interneurones and granule cells showed relatively low activity. Diffuse immunostaining of the neuropil was evident, particularly where heavy neuronal or glial staining occurred. Immunostaining was also observed in white-matter fibre tracts. This pattern of distribution helps to provide a model for the mechanisms responsible for iron homeostasis in the normal brain.