The study describes passive transfer of diabetes mellitus by transplantation of spleen cells from donor mice infected with a diabetogenic encephalomyocarditis virus. Inbred normal C57 mice were used as both donors and recipients, and other recipients were C57 athymic, nude mice. Transplants were made in two series after 12 and 22 days duration of infection of the donors, respectively. All recipients became diabetic. The possibility of virus transfer with the spleen cell transplant is discussed, and found to be highly improbable. The fact that passive cellular transfer can be effected points to a decisive significance of the thymus dependent lymphocyte system in the pathogenesis of the primary diabetes.