Blockade of CD40/CD154 pathway has proven effective in promoting the induction of allogeneic mixed chimerism. Using NOD mouse model of human type 1 diabetes, we investigated whether allogeneic mixed chimerism can be induced in prediabetic NOD mice and in spontaneously diabetic NOD mice under nonmyeloablative and irradiation-free conditioning therapy and anti-CD154 mAb as a short-term posttransplant treatment. We found that spontaneously diabetic NOD mice are more resistant to the induction of allogeneic mixed chimerism than prediabetic NOD mice under our nonmyeloablative and irradiation-free conditioning therapy. This alloresistance in spontaneously diabetic NOD mice is dependent on the CD40/CD154 pathway and donor MHC disparity.