Sphingosine 1-phosphate (S1P) is a bioactive lysophospholipid (LPL) ligand that binds endothelial differentiation gene (Edg) family G-protein-coupled receptors and has been implicated as an important regulator in endothelial cells during inflammation processes. In this study, we attempt to determine which S1P receptors mediating the inflammatory response in human endothelial cells. Our results indicated that introduction of siRNA against S1P₁ significantly suppressed S1P-induced ICAM-1 mRNA, total protein, and cell surface expressions in human umbilical vein endothelial cells (HUVECs). Moreover, U937 cells adhesion to S1P-treated HUVECs was profoundly reduced by knock-down of S1P₁ in HUVECs. By knock-down of S1P₁ or S1P₃ in HUVECs, S1P-enhanced IL-8, MCP-1 mRNA expression, and THP-1 cell chemotaxis toward S1P-treated HUVEC-conditioned media was profoundly reduced. These results suggested that S1P-induced inflammatory response genes expression is mediated through S1P₁ and S1P₃. Our findings suggest the possible utilization of S1P₁ or S1P₃ as drug targets to treat severe inflammation.