Islet transplantation can provide a therapeutic option for patients who suffer from type 1 diabetes mellitus, especially if current aggressive immunosuppression could be eliminated. In vitro immunomodulation has achieved this goal for islet allografts—however, strategies must be developed to prevent B cell lysis secondary to the chronic autoimmune process of diabetes. Previously, we have found that adjuvant therapy with CFA was effective in preventing the onset of diabetes and the recurrence of B cell lysis postislet transplantation. In this study we evaluated the efficacy of BCG, a more clinically relevant immunoadjuvant, to prevent recurrent autoimmune damage to islets grafted into diabetic NOD mice. Highly purified islets were isolated from either 5-7-week-old prediabetic NOD mice or 5-8-week-old CBA/J mice using standard islet isolation techniques. Four hundred purified islets were transplanted into the kidney capsule of diabetic NOD recipients. A single dose of BCG was administered in recipients of syngeneic (gp 1) and allogeneic islets (gp 2). In gp 1, 8 of 10 BCG-treated syngeneic recipients remained normoglycemic for> 100 days posttransplant. In untreated recipients of syngeneic islets (gp 3) the graft failed at 19 days (n= 16)(P value, gp 1 vs. gp 3< 0.001). In untreated allograft recipients, islet grafts functioned for 11 days (n= 8), which did not differ significantly (P= NS) from the BCG-treated allografts (gp 4)(14 days, n= 7). In 6 mice with long-term graft function, a second syngeneic graft implanted into the contralateral kidney maintained normoglycemia for 50 days following the removal of the first islet graft. Histological examination of the grafts from gp 1 mice showed granulated B cells with periinsular lymphocytes, while those of the control animals showed lytic B cells and marked lymphocytic infiltration. We conclude that adjuvant therapy with a single dose of BCG can prevent the recurrent autoimmune insulitis, but not allograft rejection in islets transplanted into NOD mice, and that a state of unresponsiveness is induced to allow acceptance of a second syngeneic graft. These data suggest that adjuvant therapy may be useful to sustain the function of transplanted islets in the type 1 diabetic.