Alzheimer's beta-peptides (A beta) aggregation rates depend on A beta length. We made synthetic peptide antisera to A beta 34-40 and 37-42. Purified anti-34-40 preferentially recognizes beta 1-40, vascular amyloid and a subset of plaques while purified anti-37-42 recognizes A beta 1-42 and not A beta 1-40 in dot and Western blots and immunoprecipitates; 37-42 precipitates a small percentage of fibroblast secreted A beta and strongly stains all deposits identified by monoclonals to A beta on adjacent sections from frontal cortex, but not in dorsal striatum.