Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) are both low molecular weight lysophospholipid (LPL) ligands which are recognized by the Edg family of G protein-coupled receptors. In endothelial cells, these two ligands activate Edg receptors resulting in cell proliferation and cell migration. The intercellular adhesion molecule-1 (ICAM-1, CD54) is one of many cell adhesion molecules belonging to the immunoglobulin super family. Work illustrated in this article showed that LPA and S1P enhance ICAM-1 expression at both the mRNA and protein levels in human umbilical cord vein endothelial cells (HUVECs). This enhanced ICAM-1 expression in HUVECs was first observed at 2 h post-ligand treatment. Maximal expression appeared at 8 h post-ligand treatment, as detected by flow cytometry and Western blotting. Furthermore, the effects of S1P on ICAM-1 expression were shown to be concentration dependent. Prior treatment of HUVECs with pertussis toxin, a specific inhibitor of Gi, or PDTC and BAY11-7082, inhibitors of the NFB pathway, or C. difficile Toxin B, an inhibitor of Rac, prevented the enhanced effect of S1P-induced ICAM-1 expression. However, pretreatment of HUVECs with C3, an inhibitor of rho, had no effect on S1P-enhanced ICAM-1 expression. In a static cell-cell adhesion assay system, pretreatment of LPL enhanced the adhesion between HUVECs and U937 cells, a human mononucleated cell line. The enhanced adhesion effect could be prevented by preincubation with a functional blocking antibody against human ICAM-1. These results suggest that LPLs released by activated platelets might enhance interactions of leukocytes with the endothelium through a Gi-, NFB- and possibly Rac-dependent mechanism, thus facilitating wound-healing and inflammation processes.