The effect of FK506 monotherapy on pancreaticoduodenal allotransplantation was studied in a primate model. The recipients were made diabetic by total pancreatectomy, thus glycemic control depended on the graft. In control animals hyperglycemia occurred after 14 +/- 3 days and the animals died after 36 +/- 15 days. For FK506-treated animals, the time until development of hyperglycemia was 60 +/- 12 days (P less than 0.05). The animals were then sacrificed by day 90 (P less than 0.05). All three control animals lost their graft function because of severe rejection, as verified by postmortem examination. Only one of the five treated animals showed evidence of a moderate-to-severe rejection at 90 days, one animal having a mild rejection as judged by the histological findings. The drug was clinically well tolerated in all except one animal that became apathetic and refused to eat. Hyperglycemia and an elevated serum creatinine level, which were probably related to the FK506 treatment, occurred in one animal each. Both of these side-effects were reversed by reducing the dose. Thus the use of FK506 permits successful single pancreatic transplantation in primates.