Abstract: Upon activation, platelet .alpha.-granules' soluble contents are secreted and membrane-bound contents are translocated to the plasma membrane. Membrane-bound proteins include the .beta.-amyloid precursor protein (APP) from which the .beta.-amyloid (A.beta.) deposits found surrounding the cerebrovasculature of patients with Alzheimer's Disease (AD) may originate. It has been discovered that activated platelets from AD patients exhibit less APP processing, retain more of the protein on their surface and secrete less as soluble fragments, than do controls. There is little APP or CD62 on the surface of resting platelets, but upon activation, while control platelets exhibited more of both proteins on their surface, advanced AD patients exhibited CD62 in similar amounts to controls, but retained significantly more surface APP. AD platelets secreted similar amounts of most soluble .alpha.-granule contents as controls, but less APP fragments.