Three lesions that characterize the nosologic findings in the brain of Alzheimer's presenile dementia and senile dementia of Alzheimer type, ie, neuritic plaque, neurofibrillary tangle, and microangiopathy, all are frequently associated with amyloid deposition. There has been some question, however, as to whether these lesions share the same etiology. Moreover, the specific chemical nature of amyloid associated with these lesions has not yet been determined. In the present study, formalin-fixed paraffin sections of the affected brains were tested immunocytochemically for their reactivity against antiserum to prealbumin (recently disclosed as the major constituent of amyloid associated with familial amyloidotic polyneuropathy as well as senile cardiac amyloid) and known components of other types of amyloid (AA, AP, etc.). The results demonstrated that amyloid deposits in all three lesions reacted with anti-prealbumin, suggesting that it is a common constituent of these lesions. Indeed, it is likely that prealbumin is the major constituent of amyloid associated with neuritic plaque, neurofibrillary tangle, and microangiopathy.