Abstract :LC-MS/MS allows identification of thousands of proteins in the complex proteomes. However, a significant part of a proteome remains inaccessible for identification due to the absence or poor quality of MS/MS spectra. The method described herein allows identifying the desired proteins of human blood plasma by comparing aligned chromatographic data of digested by trypsin sample and the same sample with spikedin synthetic peptides. Identification of human blood plasma proteins is archived by assigning tandem mass spectra of spiked-in peptides to the corresponding aligned chromatographic peaks of proteolytic peptides. Using the described approach we have identified 19 low abundant proteins in human blood plasma, which corresponded to 19 synthetic peptides used in the study. SRM verification of the identifications with isotopically labelled standards (SIS) confirmed the presence in the plasma of above 17 proteins.