High-efficiency nanoscale reversed-phase liquid chromatography (chromatographic peak capacities of approximately 1000: Shen, Y.; Zhao, R.; Berger, S. J.; Anderson, G. A.; Rodriguez, N.; Smith, R. D. Anal. Chem. 2002, 74, 4235. Shen, Y.; Moore, R. J.; Zhao, R.; Blonder, J.; Auberry, D. L.; Masselon, C.; Pasa-Tolic, L.; Hixson, K. K.; Auberry, K. J.; Smith, R. D. Anal. Chem. 2003, 75, 3596.) and strong cation exchange LC was used to obtain ultra-high-efficiency separations (combined chromatographic peak capacities of >10(4)) in conjunction with tandem mass spectrometry (MS/MS) for characterization of the human plasma proteome. Using conservative SEQUEST peptide identification criteria (i.e., without considering chymotryptic or elastic peptides) and peptide LC normalized elution time constraints, the separation quality enabled the identification of proteins over a dynamic range of greater than 8 orders of magnitude in relative abundance using ion trap MS/MS instrumentation. Between 800 and 1682 human proteins were identified, depending on the criteria used for identification, from a total of 365 microg of human plasma. The analyses identified relatively low-level (approximately pg/mL) proteins (e.g., cytokines) coexisting with high-abundance proteins (e.g., mg/mL-level serum albumin).