beta-amyloid (beta A, 39-43 amino acids) deposition in brain parenchyma and vessel walls is a major pathological feature of Alzheimer's disease (AD). This is associated with degenerative changes of neuronal cell bodies and processes, and neuronal death. beta A, a portion of a larger transmembrane glycoprotein, has been reported to be toxic in several tissue culture models. The neurotoxic activity of beta A synthetic peptides is associated with their fibrillogenic capacity. However, in vivo studies on beta A neurotoxic activity have not proved conclusive and further investigations are necessary to establish the pathogenetic role of beta A in AD.