The mortality rate for 80 Swiss weanlings infected with mouse hepatitis was 2.5 per cent in comparison with 98 per cent for 140 Princeton weanlings. In Swiss weanlings discrete lesions, which generally failed to progress, were observed in the liver from the 3rd through the 10th day after intraperitoneal injection. The causal virus was demonstrable in peritoneal washings through the 21st day and less regularly in the liver through the 14th day. It was also detectable in both loci after subcutaneous injection. Infant Swiss mice were susceptible through the 10th day of life, intraperitoneal injection being commonly followed by death. The pathogenicity and titer of the virus were significantly increased by successive passage in Swiss weanlings. The virus was detected in the blood of Swiss weanlings on subcutaneous injection only after it had been modified by passage.