Soluble amyloid beta-protein (A beta) is normally produced, but forms deposits in the brain of Alzheimer's (AD) patients. The length of isolated A beta in SDS-insoluble amyloid from AD brain is reported to vary between plaques and vessels ranging from 28 to 43, with major species being 40 and 42 residues. Because the A beta C-terminus determines aggregation rates, we made an A beta monoclonal antibody distinguishing beta 1-42 from beta 1-40. In AD cortex, the beta 1-42 antibody recognizes all the structures identified by monoclonal to beta 1-40 on adjacent sections: plaque cores, diffuse A beta deposits and vascular amyloid. These antibodies help define variation in A beta length between different deposits, regions and autopsied brains.