Extracellular deposition of amyloid beta protein (A beta) as senile plaques and cerebral amyloid angiopathy (CAA) is one of the essential pathological characteristics of Alzheimer's disease (AD). Several A beta species with different carboxyl termini, including A beta 42 (43) and A beta 40 ending at residue 42 (43) and 40, respectively, have been identified in CAA and in senile plaque cores. Because A beta 42 (43), the major component of diffuse plaque which is the earliest pathological change in AD brains, forms insoluble amyloid fibrils more rapidly than does A beta 40, it has been hypothesized that A beta 42 (43) plays a role in amyloid seeding and A beta 40, in the elongation of amyloid fibrils on a seed of A beta 42 (43). We used enzyme-linked immunosorbent assay (ELISA) with site-specific monoclonal antibodies to differentiate A beta 42 (43) from A beta 40. First, we measured the amounts of different A beta species in plasma from patients with sporadic probable AD, age-matched patients with neurologic diseases but without dementia, and age-matched normal controls. Concentrations of A beta 1-40 and A beta 1-42 (43) in plasma did not differ significantly among the three groups. Second, CSF levels of A beta species (CSF-A beta) with different carboxy termini, i.e., A beta X-40 and A beta X-42 (43) as well as A beta 1-40 and A beta 1-42 (43), were measured in patients with AD and in age-matched controls without dementia using ELISA. Levels of both CSF-A beta X-42 (43) and A beta 1-42 (43) were significantly lower in the patients with AD that in the controls, but neither the levels of CSF-A beta X-40 nor those of CSF-A beta 1-40 differed between the two groups, which suggest that increased adsorption of A beta 42 (43) to A beta deposition in AD brains, decreased secretion of A beta 42 (43) in CSF, or increased clearance of A beta 42 (43) from CSF might explain the low levels of A beta 42 (43) in the CSF of patients with AD. Third, we measured the concentrations of various A beta species post-mortem in the cerebral cortex of patients with PS-1 mutations and beta amyloid precursor protein (APP) 717 mutation linked to familial AD or Down syndrome. The results indicate that one effect of PS-1 mutations, APP717 mutation and Down syndrome is to cause dramatic and accelerated accumulation of A beta 42 (43) in the brain as compared with sporadic AD. In particular, the increases in A beta 1-42 (43) showed a crude inverse correlation with the age of onset in each subtype of AD. Thus, quantitative studies differentiating A beta 42 (43) from A beta 40 have established the fundamental importance of A beta 42 (43) in AD.