P-selectin is an adhesion molecule on platelets and endothelial cells that mediates the interaction of these cells with leukocytes. In addition to its docking function in cell-cell recognition, P-selectin is involved in cell signalling and cell communication. Upon platelet activation, P-selectin undergoes rapid phosphorylation and dephosphorylation. The biological role of these phosphorylation events within platelets is unknown. P-selectin, upon contact with its receptor on monocytes, initiates the de novo biosynthesis of tissue factor and other cytokines. Tissue factor upregulation on monocytes is mediated by PSGL-1 and is independent of CD14, the LPS receptor.