Electron cytochemistry was employed to identify microglial cells by nucleoside diphosphatase (NDPase) in the senile plaques of the autopsy brain in Alzheimer's disease and senile dementia. It has been elucidated that processes of microglial cells enter a senile plaque and are closely connected with amyloid accumulation. A positive reaction to NDPase revealed in the extracellular substance suggests that these cells play an important role in the accumulation of amyloid in a senile plaque.