In the clinical application of a hybrid artificial liver system using porcine hepatocytes, some immunologic reactions occur between human serum and porcine hepatocytes. In this study, we investigated the immunologic mechanisms of the cytotoxic reactions, and we tried to inactivate the human serum cytotoxicity by heating the serum or the addition of nafamostat mesilate (NM). Immunologic reaction between human serum and porcine hepatocytes by evaluating the immunochemical response against human IgM, IgG, and C3 was investigated. The immunochemical analysis of inactivation by heated human serum (56 degrees C, 30 min) and adding NM were performed. The evaluation of serum cytotoxicity was as follows: when porcine hepatocytes were cultured with heating the human serum or the addition of NM, the survival ratio was observed. Immunochemical reactions against human C3 were all positive, but positive reaction against human IgM occurred in only one case (5%); those against human IgG were all negative. Both heating the serum and adding NM inhibited the immunochemical reaction of human C3. The inhibition of human C3 with NM was dependent on that concentration. Both heating of the serum and adding NM to the medium decreased damage of porcine hepatocytes. An immunologic reaction between human serum and porcine hepatocytes in a porcine bioartificial liver clearly occurred, and this reaction was controlled by heating the serum and adding NM. We believe that NM is useful in the clinical application of our hybrid artificial liver system.