The non-obese diabetic (NOD) mouse is an inbred strain established as an animal model for the human juvenile-type of diabetes [7]. The spontaneous onset of diabetes, which is characterized by clinical findings of polyuria, polydipsia, glucosuria, hyperglycemia and rapid deterioration, is an extremely abrupt event. In the females, the symptoms first appear around the age of 13 weeks and then the incidence increases gradually with age. In the males, however, the symptoms are seen only in a few animals. The most prominent histological change shown by these diabetic mice was insulitis, ie, lymphocyte infiltration around and into the pancreatic islets. Histological investigations revealed the following two facts;(1) Insulitis preceded the onset of diabetes, occurring in high percentage of the females as early as the age of 5 to 6 weeks.(2) Insulitis was unexpectedly observed in most male mice as well, although they rarely showed any diabetic symptoms (Table 1). There-