Cell damage within the sinusoidal lining of human liver grafts during transplantation is an early event that is critical in ischemia-reperfusion injury and probably plays a key role in primary liver dysfunction after transplantation. No simple biochemical marker for sinusoidal injury is currently available. Because creatine kinase activity has been described in heart endothelial cells, we hypothesized that release of this enzyme might serve as an index of sinusoidal injury. To test this hypothesis, we used several in uivo and in uitro experimental models. Occlusion of the rat hepatic pedicle in situ for 60 min (normothermic ischemia) induced a significant increase in serum creatine kinase levels relative to those in laparotomized controls (2,530 -+ 530 vs. 389 f 64 IUL, mean f SEM; p < 0.005). In the isolated perfused rat liver, 60-min ischemia induced early ( s 3 min) creatine kinase and AST release (0.87 f 0.14 vs. 0.08 f 0.01 IU/min/gm liver, respectively). A similar phenomenon was observed after 24-hr or 48-hr hypothermic conservation in University of Wisconsin solution. Electrophoretic analysis and immunoinhibition studies showed that creatine kinase activity comprised creatine kinase-BB (=50%) and mitochondrial creatine kinase. Trypan blue infusion showed a loss of viability in sinusoidal cells, whereas hepatocytes were relatively spared. Finally, murine sinusoidal cells were isolated, cultured and then lysed by a freeze-thaw cycle and sonication. Creatine kinase activity was found in endothelial cells (creatine kinase-BB), Kupffer cells (creatine kinase-BB) and Ito cells (creatine kinase-MM). Creatine kinase-BB was not found in hepatocytes, but mitochondrial creatine kinase was detected. These results indicate that liver ischemia-reperfusion injury induces a release of creatine kinase-BB from sinusoidal cells and of mitochondrial creatine kinase from hepatocytes, suggesting that creatine kinase-BB activity could provide a simple index of cell damage within the sinusoidal lining. (HEPATOLOGY 1993;17:423-428.)