Both light and immunogold electron microscopy revealed that beta protein is closely associated with cerebrovascular and senile plaque amyloid. Diffuse (very primitive) plaques of the cerebral cortex and cerebellum in subjects with senile dementia of the Alzheimer type (SDAT) were seen as ill-defined areas of the fine fibrillar materials labeled by beta protein immunostain and silver impregnation. Electron and immunogold electron microscopic investigation revealed that cerebral diffuse plaques consist of accumulations of small bundles of amyloid fibrils scattered among the normal-looking neurites including a few altered neurites. The cell process membranes just adjacent to amyloid fibrils were frequently missing. Cerebellar diffuse plaques had structures similar to cerebral diffuse plaques, with two qualifications. First, the former had more altered neurites, and second, enlarged neurites contained mainly dense bodies and abnormally dense mitochondria lacking filamentous materials. In conclusion, cerebral diffuse plaques, which are the main plaques in SDAT, consist chiefly of amyloid fibrils interspersed among a majority of normal-looking neurites and a minority of altered neurites. This suggests the importance of amyloid deposition in the genesis of senile plaques.