Two-day-old mice were inoculated orally with a mouse hepatitis virus which had been isolated from a suckling mouse with fatal diarrhea, and a systemic infection was shown to be established, though enterotropism of the isolate was evident. Significant viral growth and syncytium forming lesions were first detectable in the intestines at day 2 postinoculation, followed by the development of focal necrotic hepatitis. The virus titer in both liver and intestine attained maximum levels at days 4 to 5. Death occurred at day 5 or later, and 60 to 100% mice died during the first 2 weeks of life. Before death, some mice showed neurologic signs with high titered viruses recovered from the brain. The necrotizing lesions were also demonstrated in the lung and lymphoid organs.