The CellKey (MDS Sciex, South San Francisco, CA) system enables comprehensive pharmacological evaluation of cell surface receptors, including G-protein coupled receptors (GPCRs) and tyrosine kinase receptors, using adherent and suspension cell lines and primary cells. A unique application enabled by the ability of the CellKey system to reliably quantify activation of endogenous receptors is receptor panning. This application allows investigators to easily screen disease-relevant cell types for functionally active target receptors by treating cells with a panel of receptor-specific ligands. Receptor panning of multiple cell types including Chinese hamster ovary, human embryonic kidney 293, HeLa, U-937, U-2 OS, and TE671 cells resulted in the identification of many functionally active, differently coupled endogenous GPCRs, some of which have not been previously documented in the literature. Upon detecting GPCR activation in live cells, unique cellular dielectric spectroscopy (CDS) response profiles are generated within minutes that reflect the signaling pathways utilized and have been shown to be characteristic of Gs, Gq, and Gi GPCRs. The fact that the CDS response profiles are predictive of the G-protein coupling mechanism of the receptor was demonstrated by using examples of subtype-selective agonists/antagonists to identify the subtypes of the endogenous histamine and beta-adrenergic receptors expressed in U-2 OS cells. A direct correlation is shown between receptor subtype G-protein coupling and CDS response profile. In addition, complex pharmacology, including detection of partial agonism and Schild analysis for endogenous receptors, is presented. The CellKey system allows investigators to conduct studies using endogenously expressed receptors to generate data that are physiologically relevant and in disease context.