Artificial liver support systems using xenoperfusion of pig or baboon liver have metabolic activity and there is the possibility that they could substitute for total liver functions; however, several problems have yet to be solved. In our early clinical experience, a method of cross-hemodialysis with interposed cuprophane membrane was employed in order to avoid immunological reactions in patients. Sixteen patients with hepatic failure were treated by this method. Although the coma grade was ameliorated in 65% of the patients, the ultimate survival rate was 18.9%. In this clinical trial, the indication for liver support was clarified based on hepatic mitochondrial functions. This unsatisfactory result could also be attributed to insufficient effects of the device, due to the interposed membrane, and also to damage of the supporting livers due to hyperacute xenoperfusion injury. Recent investigations in the field of xenotransplantations have shown us possibilities for controlling xenogeneic hyperacute rejection. Suppression of complement activation enabled long-term xenoperfusion of supporting livers with high metabolic activity. The administration of prostaglandin E1 or soluble complement receptor type 1, and the use of transgenic pig livers expressing human decay-accelerating factor, may be promising methods to establish highly active artificial liver support systems using xenoperfusion.