Measuring the affinity of a ligand for its target is an issue of central importance in life science research and drug development. In particular, understanding how well a potential drug interacts with its target (receptor, enzyme, DNA…) provides valuable knowledge in the search for new pharmaceuticals. Collective progress in chemistry, biology, robotic, informatics, led to the advent of high throughput screening (HTS) and enabled the biological evaluation of large number of drug candidates. As far as ligand-receptor system is concerned, binding of a drug candidate is characterized by a “competitive binding assay”. Since the affinity of reference ligand for target receptor is generally high, measurement of signal requires a very good sensitivity and as a consequence, radioactivity is still the method of choice for pharmacological studies of receptor/ligand systems. However, radioactivity implies a lot of constraints linked to radioelement manipulation and storage, strongly limiting high throughput screening applicability. In this context, we aim at developing a new and universal technology involving non-radioactive detection and quantification of the reference ligand displaced by the competing candidates. Mass spectrometry offers sensitive and selective methodologies to quantify molecules in complex biological systems. In contrast to known methods, the purpose of the submitted project is to develop a generic MS-based competitive binding assay avoiding the synthesis, for each evaluated molecule, of the corresponding quantification standard. This point is a great advantage since the isotopic standard used for quantification will be the same for all biological systems. The novelty of such an approach thus lies in the joint use of mass spectrometry and original labeling chemistries. Several robust mass spectrometry technologies (MALDI-MS, ESI-MS and ICP-MS) will be investigated in conjunction with different types of new chemical tags (preformed ions, elemental tags, MS enhancers…). The strategies will be validated with soluble proteins (for instance p38 kinases) as well as membrane proteins (such as two GPCR model systems: CCKB and MCR1), involving either small heterocyclic drugs or peptides as native ligands. If successful results are obtained during the course of the proposed basic research project, the developed generic tag/MS-based quantification protocol will have a great impact in biosciences, in particular in research laboratories dealing with pharmacology.