For the purpose of increasing long-term survival in canine orthotopic hepatic allotransplantation, immunosuppression by mizoribine and/or cyclosporin was instituted with the following results: 1) The survival of control dogs without treatment (n = 5) was 10.0 +/- 2.9 days (Mean +/- S.E.), and the survival of dogs treated with mizoribine (n = 15) was 44.6 +/- 31.8 days. There was no statistical difference between the two groups. On the other hand, the mean survival of dogs that were initially treated with cyclosporin and were then switched to mizoribine at one to three months after transplantation reached 145.4 +/- 70.8 days. The survival rate of dogs in the cyclosporin-mizoribine group was significantly greater than that of control group (p less than 0.02); and the survival rate of the former group proved also significantly better than that of the mizoribine group at 20 days after transplantation (X2, p less than 0.05); 2) in cases of acute rejection of the allograft, a gradual increase of the serum bilirubin level with a concomitant rise of S-GPT and alkaline phosphatase was generally observed. For an accurate diagnosis, however, an assessment of the biopsy findings of the allograft is important; 3) in chronic rejection, one animal developed the selective disappearance of interlobular bile ducts, often referred to as the "vanishing bile duct syndrome"; and 4) for differential diagnosis of vascular and/or biliary tract complications, specific morphological diagnostic procedures such as vascular and/or biliary tract angiographies are needed.