Mizoribine (Mz) is an analogue of azathioprine (Az) with less hepatotoxicity, being extensively used as immunosuppressant in place of the latter agent especially in Japan. However, careful comparative studies of mizoribine (Mz), cyclosporine (Cy), and prednisolone (Pr) versus azathioprine (Az), Cy and Pr or Cy and Pr in renal allotranspalnt patients have not been reported. Retrospectively we compared triple therapy with Mz, Cy, and Pr (group I, n = 50) to triple therapy with Az, Cy and Pr (group II, n = 13) and/or double therapy with Cy and Pr (group III, n = 11) in one-haplotype-identical living related renal transplantations performed between Oct. 1984 through March 1989. Initial and maintenance doses of Cy in groups I and II were largely two thirds of those in group III. Patient and graft survival rates at 3 years in each group are 100% and 92% (group I), 100% and 91% (group II), and 91% and 82% (group III). There were no statistical differences in patient and graft survival rates between these three groups. The incidences of miscellaneous complications were the same in the groups. Bone marrow suppression, however, was significantly less in group I than in group II (P less than 0.005). Cy related nephrotoxicity was apparently less in groups I and II than in group III. Estimated US $5,000 in a year can be saved by immunosuppressive treatment in a patient of group I as compared to a patient in group III. Therefore, we conclude that triple therapy with Mz, Cy and Pr is superior to those with Az, Cy and Pr, and/or double therapy with Cy and Pr.