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 PGE2 causes pain, vasodilation, immunosuppression of T cells, bone resorption and promotion of carcinogenesis. Four related GPCRs, EP1, EP2, EP3 and EP4, each bind to PGE2, but the different G protein coupling status of each receptor leads to distinct biological effects; EP1 couples primarily to Gq to mobilize intracellular calcium. EP1 appears to mediate the effects of PGE2 in promoting formation of precancerous lesions in animal models of colon cancer (Watanabe et al., 1999). In addition, EP1 has an inhibitory effect on stress-induced aggressive and risk-taking behaviors in mice (Matsuoka et al., 2005). EP1 membrane preparations are crude membrane preparations made from our proprietary stable recombinant cell lines to ensure high-level of GPCR surface expression; thus, they are ideal HTS tools for screening of agonists and antagonists of EP1. The membrane preparations exhibit a Kd of 14.5 nM for [3H]-PGE2. With 40 nM [3H]-PGE2,10 mg/well EP1 Membrane Prep typically yields greater than 3-fold signal-to-background ratio.