A PATHOLOGICALhallmark of Alzheimer's disease is the senile plaque, containing β-amyloid fibrils, microglia and astrocytes¹. β-amyloid fibrils exert a cytotoxic effect on neurons², and stimulate microglia to produce neurotoxins, such as reactive oxygen species^3,4. Mononuclear phagocytes, including microglia, express scavenger receptors that mediate endocytosis of oxidized low-density lipoproteins⁵, and adhesion to glucose-modified extracellular matrix proteins⁶. Here we report that class A scavenger receptors mediate adhesion of rodent microglia and human monocytes to β-amyloid fibril-coated surfaces leading to secretion of reactive oxygen species and cell immobilization. Thus, class A scavenger receptors are potential therapeutic targets in Alzheimer's disease.