Lysophosphatidic acid(LPA) is a novel growth-factor-like lipid mediator. It can be released from stimulated platelets as a product of the blood-clotting process. The number and diversity of the known biological responses to LPA keep growing from many years ago when it was first found as an out-membrane messenger. The more important function of LPA is that it can induce proliferation of many types of cells. Several LPA receptor cDNA clones have been found. LPA influences target cells mainly by activating special G protein coupled receptors. The signal transduction system of LPA includes several known signal cascades: activating G_q to stimulate PLC; activating G_i to restrain adenylyl cyclase and stimulate MAPK cascade; and activating G_12/13 to stimulate Rho cascade, etc.