Complement-activated oligodendroglia (CAOs) are thought to represent complement bearing damaged oligodendroglia for opsonization. The interrelationship between CAOs and amyloid deposits was examined by immunohistochemistry in the parietal lobe of patients with Parkinson's disease, diffuse Lewy body disease, and pallido-nigro-luysial atrophy. In all brains, the anti-C4d antibody stained numerous CAOs. Anti-beta-amyloid protein (anti-A beta) antibody revealed moderate numbers of senile plaques, including some of the classical type. In both the grey and the white matter amyloid deposits were frequently associated with the myelinated axons of CAOs. CAOs were occasionally associated with phagocytosing microglial cells. Immunoelectron microscopy also showed a close relationship between phagocytosing microglia and A beta deposition. On some occasions. A beta deposits were seen in C4d-positive oligodendroglial cell bodies. These results indicate that damaged myelinated axons, which contain accumulated amyloid precursor protein, are the source of A beta, and that CAOs may be initial targets for A beta deposits forming the classical senile plaques.