The lysophospholipid growth factors sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) are generated by many cells involved in immunity, including macrophages, dendritic cells, mast cells, and platelets, with resultant lymph and plasma concentrations of 0.1-1 microM. All immune cells express distinctive profiles of G protein-coupled receptors (GPCRs) for S1P and LPA, which are regulated developmentally and by cellular activation. For T-cells, constitutive S1P signaling through their principal S1P(1) GPCR inhibits chemotactic responses to chemokines, with lesser suppression of proliferation and cytokine production. These S1P-S1P(1) GPCR signals tonically reduce T-cell chemotactic sensitivity to chemokines and thereby limit homing of blood and spleen T-cells to secondary lymphoid tissues. S1P(1) GPCR antagonists evoke lymphopenia by permitting blood T-cells to enter lymph nodes and blocking S1P(1) GPCR-dependent T-cell efflux from lymph nodes. Inversely, there is a longer than normal persistance in blood and a decrease in lymphoid transit time for T-cells overexpressing transgenic S1P(1) GPCRs. The immunotherapeutic potential of S1P(1) GPCR antagonists derives from their capacity to limit T-cell access to organ grafts and autoimmune antigens without reducing their other intrinsic functional capabilities. Lysophospholipids and their GPCRs thus constitute an immunoregulatory system of sufficient prominence for pharmacological targeting in transplantation, autoimmunity and immunodeficiency.