Recovery from hyperglycaemia was observed in three different mouse hybrids that were made diabetic with streptozotocin, and then transplanted with pancreas from isogeneic donors whose pancreatic ducts were ligated over 8 weeks earlier. Recovery did not occur in recipients of allografts indicating that islets in ligated pancreas cannot be successfully transplanted across a major histocompatibility barrier. However, in lethally irradiated mice injected with allogeneic bone marrow (allogeneic radiation chimeras), subsequent transplantation of ligated pancreas from either isogeneic donors or allogeneic donors of the same strain as the bone marrow donor decreased blood glucose of diabetic recipients. Thus, irradiation and allogeneic bone marrow therapy allow subsequent allogeneic transplantation of functional islet tissue in diabetic mice. Observations on diabetic mice transplanted isogeneically with ligated pancreas suggest that recovery from hyperglycaemia was more consistent when grafts were placed subcutaneously rather than intraperitoneally and when hosts received ligated pancreas from three donors rather than one. Thus, the site and amount of tissue transplanted are important considerations in the transplantation of ligated pancreas. Persistence of endocrine beta-cells in ligated pancreas was required for recovery in grafted mice since diabetic hosts failed to recover when grafted with ligated pancreas obtained from alloxan or streptozotocin-treated, diabetic donors.