Proteinase-activated receptors (PARs) are a novel subclass of seven transmembrane-spanning, G protein-coupled receptors. PAR-1 and PAR-2 are widely expressed in a variety of cells and are found to be involved in many physiological and pathological processes including inflammation and immune response. However, little is known about the function of PAR-1, 2 in acute graft host disease (GVHD). In the present study, we first detected the expression of PAR-1, 2 protein and mRNA in a murine model of acute GVHD using the methods of immunohistochemistry, Western blot and quantitative real-time polymerase chain reaction (PCR). Syngeneic hematopoietic stem cell transplantation (HSCT) mice served as controls. The relative gene expression level of PAR-1 was significantly increased in the skin, liver, small intestine of allogeneic HSCT mice (in skin: 0.039±0.013 0.008±0.002 of controls, =0.009; in liver: 0.165±0.006 0.017±0.006 of controls, =0.004; in small intestine: 0.215±0.009 0.016±0.002 of controls, =0.003), but not in the stomach, lung and kidney of allogeneic HSCT mice (>0.05). PAR-2 mRNA expression in the liver and small intestine of allogeneic HSCT mice (in liver: 0.010±0.002 0.003±0.001 of controls, =0.008; in small intestine: 0.006±0.001 0.003±0.001 of controls, =0.024) was increased significantly, but PAR-2 mRNA expression in the other organs (>0.05) was not found to be significantly elevated. PAR-1, 2 protein expression was in accordance with the mRNA expression, as shown by Western blot. Using immunohistochemistry the present study demonstrated that there was strong PAR-1, 2 immunoreactivity in the epithelial cell and vascular endothelial cell of target organs of acute GVHD. Our findings of markedly increased expression of PAR-1, 2 in target organs of acute GVHD suggest that PAR-1 and PAR-2 may play an important role in the pathogenesis of acute GVHD.