alpha 1-Antichymotrypsin, a member of the serine proteinase inhibitor (serpin) family, inhibits neutrophil proteinase cathepsin G and mast cell chymases, and protects the lower respiratory tract from damage by proteolytic enzymes. It contains a reactive centre loop, which interacts with cognate proteinases, resulting in loop cleavage and a major conformational change. Recently, alpha 1-antichymotrypsin has been identified as a major constituent of the neurofibrillary plaques associated with Alzheimer's disease, and in vitro studies have shown that it enhances the rate of amyloid-fibril formation. These observations and recent genetic evidence suggest that alpha 1-antichymotrypsin is important in the pathogenesis of Alzheimer's disease.