Prostanoids are a series of arachidonic acid metabolites produced by the action of cyclooxygenase and subsequently by isomerases and synthases. Cells rapidly secrete prostanoids after synthesis, whereupon the prostanoids bind to a family of 8 GPCRs to exert their biological effects (Narumiya and FitzGerald, 2001). The prostaglandin PGD2 is produced by mast cells upon activation by allergens, and is present at high levels in allergic diseases. PGD2 binds to two receptors, DP and CRTH2. DP activates Gs to increase cAMP levels, and lack of DP results in reduced allergic response in animal models of bronchial asthma (Matsuoka et al., 2000). The cloned human DP-expressing cell line is made in the Chem-1 host, which supports high levels of recombinant DP expression on the cell surface and contains high levels of the promiscuous G protein Gα15 to couple the receptor to the calcium signaling pathway. Thus, the cell line is an ideal tool for screening for antagonists of interactions between DP and its ligands.