Introduction: Antithrombin (AT), an important inhibitor of the coagulation system, promotes endothelial release of PGI2 by interacting with cell surface heparin-like substances. The AT-induced promotion of PGI2 release from endothelial cells has been shown to be important in preventing the activated leukocyte-induced endothelial injury in rats given endotoxin. In the present study, we examined whether AT reduces ischemia/reperfusion (I/R)-induced liver injury by inhibiting leukocyte activation. Methods: Hepatic damage was induced in male Wister rats by 60 min-ischemia and the subsequent reperfusion. We measured serum transaminases levels (n = 40), hepatic 6-keto-PGF1α (n = 40) levels, cytokine-induced neutrophil chemoattaractant (CINC; equivalent as human IL-8) levels (n = 40), and hepatic myeloperoxidase (MPO) acitivity (n = 40). Results: Although AT (250 U/kg) markedly reduced hepatic injury, neither an inactive derivative of factor Xa (DEGR-Xa), a selective inhibitor of thrombin generation, nor Trp49-modified AT which lacks affinity for heparin had any effect. AT significantly enhanced the I/R-induced increase in hepatic level of 6-keto-PGF1α, while neither DEGR-Xa nor Trp49-modified AT enhanced it. AT significantly reduced I/R-induced increase in hepatic levels of CINC and MPO, but either DEGR-Xa or Trp49-modified AT had no effects. Iloprost, a stable analog of PGI2, exhibited effects similar to those of AT. Treatment GOT (IU//L) 6-keto-PGF1α (ng/g) MPO (U/g) I/R (n=6) 5285 ± 547 5.4 ± 0.3 1.20 ± 0.09 AT (n=6) 2838 ± 199*6.9 ± 0.7*0.93 ± 0.10*DEGR-Xa (n=6) 5160 ± 336 5.3 ± 0.6 1.17 ± 0.19 Modified AT (n=6) 6316 ± 621 5.1 ± 0.4 1.21 ± 0.15 Iloprost (n=6) 2622 ± 176*not determined 0.89 ± 0.14*[Data are expressed as mean ± SD.*, p<.01 vs. I/R. The results were compared using an analysis of variance followed by Scheffé's (post hoc) test] Conclusions: These findings strongly suggest that AT may prevent I/R-induced hepatic injury by increasing the hepatic levels of PGI2 through the interaction of AT with cell-surface glycosaminoglycans, thus inhibiting leukocyte activation in animals subjected to I/R.