Abstract: This invention describes a specific, sensitive, reproducible and multiplexed assay for simultaneous quantification of full-length beta-amyloid peptides in biological matrices; a method to monitor and measure the hallmarks associated with the progress of Alzheimer's disease. This invention employs one specific antibody that recognizes the N-terminus of all the abeta peptides, and a panel of detection antibodies that distinguish each abeta peptide by their sequence difference at the C-terminus. Each of these C-terminus specific antibodies carries a different label (or tag), for example, fluorescence labels with different excitation/emission characteristics or electrochemiluminescence (ECL) label with different excitation/emission characteristics. This invention allows simultaneous quantification of several abeta peptides in one single assay. The concept of using N- and C-terminus specific antibodies to capture the each of the said abeta peptides by both ends is the basis of providing the desired specificity.