NOR/Lt is an insulitis-resistant and diabetes-free strain produced from an isolated genetic contamination within an NOD/Lt pedigree line. The albino coat-color phenotype, strain-specific endogenous retroviral profile, and skin graft tests indicated an NOD/Lt × C57BL/KsJ outcross-backcross segregant as the source of the contaminating genome. Analysis of 53 polymorphic DNA, biochemical, and immunologic markers distinguishing NOD/Lt from C57BL/KsJ revealed that 4 chromosomes (chromosomes 2, 4, 11, and 12) in NOR/Lt contained C57BL/KsJ-derived genes. The remaining markers on 14 chromosomes, including the diabetogenic H-2^g7 complex on chromosome 17, were of NOD origin. Although completely resistant to cyclophosphamide-induced diabetes, NOR/Lt mice exhibited the same peripheral T-lymphocyte accumulation characteristic of NOD/Lt. Similarly, NOR/Lt peritoneal macrophages exhibited depressed interleukin-1 secretion characteristic of NOD/Lt. In addition to their diabetes resistance, NOR/Lt mice were distinguished from NOD/Lt by exhibiting more robust suppressor T-lymphocyte function. Outcross of NOR/Lt with NOD/Lt to generate heterozygosity at those chromosomal segments, defined by C57BL/KsJ markers in NOR/Lt parentals, did not produce insulitis or diabetes in F₁ females. However, these F₁ females were sensitive to cyclophosphamide-induced diabetes. In summary, the NOR/Lt strain is an MHC-matched diabetes-resistant control strain for NOD/Lt. Moreover, NOR/Lt will help identify the location and function of a non-MHC gene or genes capable of conferring resistance against insulitis and diabetes.