Amyloid-beta (A beta) peptide oligomerization plays a central role in the pathogenesis of Alzheimer's disease (AD), and A beta oligomers are collectively considered an appealing therapeutic target for the treatment of AD. However, the molecular mechanisms leading to the pathologic accumulation of oligomers are unclear, and the exact structural composition of oligomers is being debated. Using targeted and quantitative mass spectrometry, we reveal site-specific A beta autocleavage during the early phase of aggregation, producing a typical A beta fragment signature and that truncated A beta peptides can form stable oligomeric complexes with full-length A beta peptide. We show that the use of novel anti-A beta antibodies raised against these truncated A beta isoforms allows for monitoring and targeting the accumulation of truncated A beta. fragments. Antibody-enabled screening of transgenic models of AD as well as human postmortem brain tissue and cerebrospinal fluid revealed that aggregation-associated A beta cleavage is a highly relevant clinical feature of AD. (C) 2016 The Alzheimer's Association. Published by Elsevier Inc. All rights reserved.