Nonobese diabetic (NOD) mice are the experimental prototype of type 1 insulin-dependent diabetes mellitus (IDDM). These mice develop a characteristic autoimmune lesion in the pancreatic islets of Langerhans, where infiltrating lymphocytes destroy beta cells, resulting in hypoinsulinemia, hyperglycemia, ketoacidosis, and death. This IDDM, which closely resembles that in humans, is prevented by infecting NOD mice with particular strains of lymphocytic choriomeningitis virus (LCMV), including Armstrong 53b, Traub, WE, and Pasteur. In contrast, the LCMV Armstrong 53b variant, Clone 13, fails to abort IDDM. Hence, although Clone 13 establishes a persistent infection that endures throughout the life spans of NOD mice, their hyperglycemia, hypoinsulinemia, and lymphocytic infiltration into the islets of Langerhans still occur. Genetic reassortant viruses generated between the IDDM therapeutic strain of LCMV Pasteur and the nontherapeutic variant, LCMV Clone 13, were used to treat NOD mice. By using such reassortants and both parental strains of virus to infect NOD mice, the prevention of IDDM was mapped to the S RNA segment of LCMV Pasteur.