Reovirus type 2 that had been isolated from a cow with diarrhoea and passaged in bovine kidney cell culture produced a Type 1 (insulin-dependent) diabetes-like syndrome when inoculated into NC mice. The infection resulted in insulitis and destruction of islet cells. Viral antigens were found in islet cells by staining with fluorescein-labelled antibody to reovirus type 2. The destruction of islet cells resulted in abnormalities shown on glucose tolerance testing. Studies on the susceptibility of the host showed that only certain strains of mice had overtly abnormal glucose tolerance tests when infected with reovirus type 2. To assess the immunological role in the pathogenesis of reovirus type 2-induced diabetes, infected mice were subjected to immunosuppressive or thymic hormone treatment. The administration of either anti-thymocyte serum or serum thymic factor reduced or prevented the development of the diabetes-like syndrome, while Arg-Lys-Asp-Val-Try did not show any therapeutic effects.