A monoclonal antibody raised to a synthetic peptide consisting of residues 8 to 17 of the amyloid beta protein of Alzheimer's disease was employed for immunogold electron microscopic studies on amyloid fibrils of cerebrovascular walls and neuritic plaques in this disease. Electron microscopy revealed a specific gold labeling of the amyloid fibrils in these structures. This provides ultrastructural evidence that beta protein is intimately associated with the amyloid fibril. With previous chemical evidence, this observation supports the concentration that it is an intrinsic component of the fibril.