Islet transplantation for the treatment of autoimmune diabetes is more difficult because of the additional barrier presented by the autoimmunity. We tested the ability of hamster anti-rat CD154 to prevent recurrence of diabetes in renal subcapsular islet isografts in DR-BB (RT1uu) rats with established autoimmune diabetes. Experimental animals with established diabetes received intravenous injections of 15 mg/kg anti-CD154 on a specified schedule starting 2 days before renal subcapsular transplantation of an islet isograft. Control animals received either saline or hamster IgG. Plasma glucose levels >250 mg/dl over 3 days were used to indicate the recurrence of diabetes. Rats that received saline (n = 5) or control antibody (n = 3) had a recurrence of diabetes 6-11 days after transplantation. Histological examination of islet isografts from these rats showed complete destruction of the insulin-producing portion of the isograft with residual cells positive for glucagon. Recipient rats that received anti-CD154 at the 15-mg/kg dosage (n = 6) did not have a recurrence of diabetes for 308-461 days after transplantation. Islet isografts removed from the rats showed low levels of insulin immunoreactivity, high levels of insulin mRNA, and focal infiltration with lymphocytes but no evidence of islet destruction. Mean peak antibody concentration was 266 μg/ml and returned to undetectable levels by 67-88 days after transplantation. Rats that received anti-CD154 starting at 4-7 days after transplantation had a recurrence of diabetes within 11 days of the isotransplantation. Therefore, anti-CD154 as the sole immunomodulator prevented the recurrence of diabetes in islet isografts in rats with established autoimmune diabetes. This suggests that CD40/CD154 blockade is effective in preventing the insulitis or the effector phase of autoimmune diabetes.