G protein coupled receptors (GPCR) are a most prominent group of therapeutic targets. In most cases, the activation of GPCRs results either in the alteration of cellular cAMP level or in the release of calcium ions from intracellular stores. Various assays are available to determine receptor activation by direct or indirect quantifi cation of these second messengers. Cisbio Bioassays developed highly accurate HTRF® assays for measuring second messengers like IP1 or cAMP in HTS formats. HTRF® (homogeneous time-resolved fl uorescence) is based on a fluorescence resonance energy transfer between a Europium cryptate (donor) and a second fl uorescent label (acceptor). This application note demonstrates the use of the IP-One HTRF® assay for a functional selection of clones from stably transfected recombinant CHO-M1 cells. The measurements were performed on BMG LABTECHs PHERAstar.