A quantitative method for detection of amyloid beta peptides using immunoprecipitation-HPLC-mass spectrometry (IP-LC-MS) is described. Comparison of IP-LC-MS with sandwich ELISA revealed comparable results in the analysis of A beta 1-40 and A beta 1-42 derived from fetal guinea pig cell media and cell lysates. The use of IP-LC-MS not only allows a quantitative method for A beta 1-40 and A beta 1-42 peptides present in Alzheimer's disease (AD), but allows detection of other A beta peptide species that may also play a role in the onset of AD in humans.