Sphingosine 1-phosphate (S1P) is a bioactive lipid that binds to and activates a family of GPCRs, S1P1-5 (also known as EDG receptors). Interactions between S1P and its receptors mediate cytoskeletal rearrangement and cell migration, with functional consequences in angiogenesis, lymphocyte trafficking, and smooth muscle development (Anliker and Chun, 2004). S1P1 (Edg-1) signals exclusively through Gi, whereas S1P2 (Edg-5) and S1P3 (Edg-3) activate Gi, Gq and G12/13 (Windh et al., 1999). Although S1P1 and S1P3 promote cell migration, S1P2 inhibits cell migration in several cell types; these opposing functions appear to result from differences in the ability of each receptor to activate Gi (Sugimoto et al., 2003). Studies with knockout mice indicate that S1P2 and S1P3 have redundant functions in maintaining vascular integrity during embryonic development (Kono et al., 2004). In addition, S1P3 regulates immune responses by contributing to endothelial barrier function in splenic marginal zones (Girkontaite et al., 2004). Millipore's S1P3 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 S1P3 interactions with its ligands. The membrane preparation exhibits EC50 of 1.7nM for S1P in a GTPγS binding assay.