A polyclonal antibody to a synthetic peptide representing a 28-amino-acid sequence of the previously isolated and described Alzheimer's disease amyloid precursor was raised in rabbits. The antibody was used in conjunction with an avidin-biotin-peroxidase technique to stain cerebral microvessels involved by amyloid angiopathy and senile "neuritic" plaque amyloid cores. The staining method has significant advantages over standard histologic techniques used to demonstrate brain amyloid and might have important practical applications in the study of microvascular lesions associated with cerebral amyloid angiopathy, as well as in studies on the pathogenesis of Alzheimer's disease or senile dementia of Alzheimer type.