The M variant of encephalomyocarditis virus produces a diabetes mellitus-like disease in DBA/2 mice but not in animals of the C3H strain. Fewer than one-third of infected F1 (DBA/2 x C3H) progeny exhibit the disease, whereas the prevalence in backcrosses (F1 x DBA/2, F1 x C3H) is comparable to the parental inbred strain. Thus, the mode of inheritance of the diabetic predisposition appears to be polygenic. DBA/2 animals develop striking inflammatory and necrotizing lesions of the islets of Langerhans; in contrast, alterations of the insular tissue in the C3H mice are minimal. Although metabolic abnormalities appear to be consequent to lesions of beta cells, the factors influencing the severity of these insular changes are incompletely understood.