Two distinct species of amyloid beta protein (A beta) with different C-termini, A beta 42(43) and A beta 40, are deposited in senile plaques (SP) of Alzheimer's disease (AD), with the former being far predominant. To investigate whether A beta 42(43) also predominates over A beta 40 in normal aging, we examined by immunocytochemistry the C-termini of A beta in SP in the brains of non-demented aged individuals and compared the results with those in AD. Virtually all SP were A beta 42(43)-positive; of these 12% in non-demented aged individuals and 25% in AD patients (mean of three areas examined) were also A beta 40-positive. In both the AD and non-demented groups, 2/3 of the A beta 40-positive SP were typical cored SP. These results indicate that A beta 42(43) is the predominant species deposited in SP in normal aging, and there is no qualitative difference in terms of the C-terminus of A beta in the parenchymal amyloid deposition between normal aging and AD.