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). Chemicon's cloned human S1P1-expressing cell line is made in the Chem-1 host, which supports high levels of recombinant S1P1 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 S1P1 and its ligands.