The presence of amyloid precursor protein (APP) and beta-amyloid protein (beta A4) was investigated in the cerebra of 4 patients with Alzheimer's disease (AD), 1 patient with Down's syndrome, 4 patients with dementia of the Lewy body type (DLB) and 4 age-matched, clinically nondemented controls, of which one displayed many amyloid plaques. The different types of amyloid plaques stained strongly with antibodies against beta A4. Antibodies against the C-terminal region of APP reacted only weakly with small swollen neurites and with globular deposits in neuritic-type plaques from patients with AD. The antibody against the N-terminal region of APP stained strongly cellular elements in the neuritic type plaques of patients with AD but not dense cored plaques. In contrast, patients with DLB displayed with this antibody a homogeneous staining of dense cored amyloid plaques. Some Lewy bodies stained with the antibody against the N-terminal region of APP as well. These results indicate that the processing of APP in AD and DLB could be different, to yield different fragments deposited in AD and DLB amyloid plaques.