Chemoattractant receptor homologous molecule expressed on Th2 cells (CRTH2) is a GPCR related to FPR-like chemoattractant receptors that is highly expressed on Th2 cells, eosinophils, basophils, and type 2 cytotoxic T cells (Nagata et al., 1999). The most potent ligand for CRTH2 is prostaglandin D2 (PGD2), a major mast cell-derived allergic mediator (Hirai et al., 2001). Another GPCR, DP, is activated by PGD2, and the relative contributions of DP and CRTH2 to PGD2-mediated allergic disease are being elucidated. A small molecule antagonist of CRTH2, ramatroban, attenuates PGD2-mediated bronchial hyperresponsiveness and antigen-induced inflammation (Sugimoto et al., 2003; Shichijo et al., 2003). The cloned human CRTH2 expressing cell line is made in the Chem-5 host, which supports high levels of CRTH2 expression on the cell surface and contains high levels of the promiscuous G proteins to enhance coupling of the receptor to the calcium-signaling pathway. The cell line is an ideal tool for screening antagonists of interaction between CRTH2 and its ligand.