Fifty hand-picked islets were freshly prepared from DBA/2 mice and transplanted four times into a streptozotocin-induced diabetic B6AF1 mouse without immunosuppression. Blood sugar levels decreased progressively and all recipients became normoglycemic after the 4th grafting. Four repeated transplantations at 2-4-day or 14-day intervals restored normoglycemia for more than 200 days in virtually all recipients. However, a majority of recipients given four transplantations of 50 DBA/2 islets at 7-day intervals or four transplantations of 100 DBA/2 islets at 4-day or 14-day intervals acutely rejected their grafts. When handpicked islets were freshly prepared from each of four histoincompatible donors (DBA/2, DBA/1, A.SW, and C3H) and sequentially transplanted four times (islets from one donor for each transplantation) into diabetic B6AF1 recipients, virtually all recipients maintained normoglycemia for more than 200 days regardless of the number of islets per grafting or intervals between transplantations. Since the purity of human islet preparations to date has been at the level of crude-digested islets, crude islets were used in sequential transplantation. Four transplantations of approximately 50 crude islets prepared from each of four donors achieved marked prolongation of graft survival. In sharp contrast, transplantation of 250-500 single-donor crude islets or four sequential transplantations of DBA/2 50 crude islets resulted in acute graft rejection.