Fibrillar amyloid beta protein (A beta) deposition is increased in the brains of patients with Alzheimer's disease (AD), and is manifested as senile plaques (SPs) and congophilic angiopathy (CA). A beta 40 and A beta 42(43), two chief species of A beta, are documented in SPs and CA, as well as in cerebrospinal fluid (CSF) and cell culture media. A beta 42(43) is the major component of diffuse plaques, the earliest form of SPs. Thus, we hypothesized that determination of the amount of A beta 42(43) in CSF or plasma might provide a diagnostic laboratory test for AD. We measured amounts of different A beta species in plasma from 28 patients with sporadic probable AD, 40 age-matched neurologic patients without dementia and 25 age-matched normal controls using enzyme-linked immunosorbent assays (ELISAs). Plasma concentrations of A beta 1-40 and A beta 1-42(43) did not significantly differ among these groups. These findings suggest the unlikelihood that plasma A beta assays would be useful as a diagnostic tool for AD.