The cerebral and vascular plaques associated with Alzheimer's disease (AD) are mainly composed of amyloid beta peptides (Ab). Ab is derived from cleavage of the amyloid precursor protein (APP) and varies in length from 39 to 43 amino acids. Ab [1-40], Ab [1-42], and Ab [1-43] peptides result from cleavage of APP after residues 40, 42, and 43, respectively. The cleavage takes place by gamma-secretase during the last APP processing step. Ab [1-40], [1-42] and [1-43] peptides are major constituents of the plaques and tangles that occur in AD. Ab antibodies and peptides have been developed as tools for elucidating the biology of AD.