Aß (1-42), a major component of amyloid plaques, accumulates in neurons of Alzheimer’s disease brains. Biochemical analysis of the amyloid peptides isolated from Alzheimer’s disease brain indicates that Aß (1-42) is the principal species associated with senile plaque amyloids, while Aß (1-40) is more abundant in cerebrovascular amyloid deposit. This peptide is biotinylated at the N-terminus for convenient detection and purification.