BB rats serve as a model for human insulin-dependent diabetes mellitus (IDDM), since without insulin treatment, most 60-140-d-old animals die within 1 to 2 wk of developing polyuria, polydypsia, hyperglycemia, and hypoinsulinemia. Lymphoid cells accumulate in the islets of Langerhans and beta cells undergo destruction. We report that inoculation of such BB rats with lymphocytic choriomeningitis virus (Armstrong strain, clone 13) reduces over a prolonged period the incidence of IDDM, normalizes the concentration of blood sugar and pancreatic insulin, prevents the mononuclear cell infiltration in the islets of Langerhans, and for a short time after inoculation alters T lymphocyte subsets. Thus, a virus might be programmed to carry out useful functions.