Physiological concentrations (1g/L) of Low density lipoprotein (LDL) enhance platelet aggregation responses initiated by thrombin, collagen, and ADP. This enhancement involves the rapid phosphorylation of p38 mitogen-activated protein kinase (p38MAPK) at Thr180 and Tyr182. The receptor for LDL is ApoER2, a splice variant of the classical ApoE receptor. ApoER2 stimulation leads to association of the Src family kinase Fgr which is probably responsible for subsequent phosphorylation of p38MAPK. This stimulation is transient because LDL also increases the activity of PECAM-1, which stimulates phosphatases that dephosphorylate p38MAPK.