Sphingosine 1-phosphate (S1P) is a biologically active lysophospholipid that transmits signals through a family of five G-protein-coupled receptors to regulate cell proliferation, migration, cytoskeletal organization, and differentiation. S1P1 was the first identified S1P receptor. It primarily couples to PTX-sensitive Gi/o proteins and mediates S1P-induced adenylate cyclase inhibition. Expression of S1P1 is pervasive, including spleen, brain, heart, lung, adipose tissues, liver, thymus, kidney, and skeletal muscle (Zhang et al. 1999). The deletion of S1P1 in mice results in embryonic lethality (Liu et al., 2000) with death attributable to incomplete vascular maturation. Recent reports demonstrate specific roles for S1P1 in lymphocyte recirculation/egress (Matloubian et al., 2004). Millipore's S1P1 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 S1P1 interaction with its ligands. The cell line exhibits a calcium response with EC50s of 141 μM for Sphingosine 1-phosphate. The membrane preparations exhibit EC50s of 9.5 μM for Sphingosine 1-phosphate in a GTPγS binding assay.