Alzheimer senile plaques are comprised of extracellular deposits of beta-amyloid. The cell type responsible for the secretion of this amyloid, however, is unknown. In the present study, compact amyloid 'stars' and the cellular elements directly apposed to them were examined at the ultrastructural level. In many cases, amyloid fibrils were closely interdigitated with the plasma membrane of cells with dark cytoplasm, dense bodies and distinctive nuclei. These cells were morphologically identified as microglia, the resident macrophages of the brain. Previous work has described an identical morphologic association between macrophages and several types of systemic and cerebral amyloidoses. Taken together, these data suggest that beta-amyloid may be secreted by microglia.