Abstract: The present invention provides compositions and methods for diagnosing Alzheimer's disease (AD). In particular, the present invention provides monoclonal antibodies that specifically bind to soluble, non-fibrillar oligomeric amyloid ? protein assemblies proteolytically derived from the transmembrane amyloid precursor protein (APP) while not reacting with fibrillar amyloid ? protein assemblies, monoclonal antibodies that specifically bind to fibrillar amyloid ? protein assemblies that do not react with soluble, non-fibrillar oligomeric amyloid ? protein assemblies, and methods of use of these compositions in the diagnosis of Alzheimer's disease, as well as methods to monitor treatment and/or disease progression of AD in patients.