In the pharmaceutical industry, various compounds such as antibodies, peptides, oligonucleotides, etc. are being evaluated and developed as new modality drugs as well as small molecule compounds. For the evaluation of these new modalities, the development of analytical methods is essential, but often difficult because the analytical techniques related to these modalities are not as mature as to small molecule compounds. Liquid chromatography/mass spectrometry (LC/MS) is a highly selective and sensitive method with a wide dynamic range, and has become an indispensable instrument for analysis of small molecule compounds. Recently, LC/MS has attracted attention as an alternative to ligand-biding assays for the quantification of peptides and proteins. So far, we have found that the adsorption capacity of a peptide changes abruptly and reversibly around a specific organic solvent content (critical point) in the solution, and based on this property we developed a peptide adsorption-controlled liquid chromatography (PAC-LC) for quantitative peptide analysis. By using PAC-LC, it has become possible to quantify low concentration proteins and peptides in biological samples. In this symposium, I would like to introduce examples of analysis for biomarker proteins and peptides in biological samples using PAC-LC.