In the NOD mouse, an autoimmune process beginning by 5 weeks of age with lymphocyte infiltration and destruction of insulin-secreting beta cells leads to overt diabetes which begins to appear by 11 weeks of age. Although there is a high incidence of insulitis by 10 weeks of age (greater than 80%) in both males and females, by 30 weeks of age diabetic symptoms have occurred in 53-80% of females and in 12-40% of males. Intraperitoneal injection of a high dose (200 mg/kg) of cyclophosphamide (CY) consistently induces the onset of diabetes in male and female NOD mice at an age when spontaneous diabetes rarely occurs. Spleen T cells from CY-induced diabetic mice are capable of transferring the disease into irradiated nondiabetic syngeneic recipients. This indicates that the diabetogenic effect of CY is not mediated by direct toxicity on pancreatic beta cells but is mediated by abrogation of a suppressor mechanism which may prevent activation of T cells responsible for the development of diabetes in the NOD mouse. Additionally, CY is only effective in NOD mice and not in F1 hybrids between NOD and other strains of mice. Thus, the potential beta cell aggressor mechanism is not present in these hybrids as it is in homozygous mice, which indicates that it is not under the control of dominant genes.