Coxsackie virus B4 that had been passaged in cultures enriched for pancreatic beta cells produced a diabetes-like syndrome when inoculated into SJL/J mice. The infection resulted in insulitis and destruction of beta cells. Viral antigens were found in beta cells by staining with fluorescein-labeled antibody to Coxsackie virus B4. The destruction of beta cells led to a decrease in the immunoreactive insulin content of the pancreas and hypoinsulinemia. The reduction in immunoreactive insulin correlated inversely with the elevation of glucose in the blood and over 80% of the animals were found to be hyperglycemic within 14 days after infection. The percentage of animals with hyperglycemia decreased with time and at the end of 60 days, less than 5% of the animals were still hyperglycemic. However, many of the normoglycemic mice were found to be metabolically abnormal when evaluated by glucose tolerance tests. Studies on the susceptibility of the host showed that only certain inbred strains of mice became diabetic when infected with Coxsackie virus B4. It is concluded that both the passage history of the virus and the strain of the host influence the development of diabetes.