Plasma Proteome Institute View Institution's Website 1 article published in JoVE Biology Quantification of Proteins Using Peptide Immunoaffinity Enrichment Coupled with Mass Spectrometry Lei Zhao*1, Jeffrey R. Whiteaker*1, Matthew E. Pope2, Eric Kuhn3, Angela Jackson4, N. Leigh Anderson5, Terry W. Pearson2, Steven A. Carr3, Amanda G. Paulovich1 1Clinical Research Division, Fred Hutchinson Cancer Research Center - FHCRC, 2Department of Biochemistry and Microbiology, University of Victoria, 3Broad Institute of MIT and Harvard, 4Genome BC Proteomics Centre, University of Victoria, 5Plasma Proteome Institute Stable Isotope Standards and Capture by Anti-Peptide Antibodies (SISCAPA) couples affinity enrichment of peptides with stable isotope dilution mass spectrometry (MRM-MS) to provide quantitative measurement of peptides as surrogates for their respective proteins. Here we describe the protocol using magnetic particles in a partially automated format.