The chronic infection was established in the mouse by the inoculation of the virus isolated from the patients of infectious hepatitis in Okayama prefecture, and the infectious course was investigated pathohistologically for about 500 days. The results are summerized as follows; 1) Though most of the mice inoculated with the virus did not succumb but took a chronic course, the typical pathological findings were observed with the liver as the focus. With the laps of time, however, some of the mice died, in which the pathological changes were particularly marked. 2) The general pathological findings were the degeneration and necrosis of the parenchymal cells and the cell infiltration in the liver. These findings varied with the laps of time and the cell infiltration became marked enough to be a sign of infection, though the degeneration and necrosis were still somewhat obserbed. 3) In the lung, there were observed the interalveolitis or the proliferation of round cell or mesenchymal cell around bronchi and blood versels, which was considered to be caused by the infection of the virus. 4) The reversion of the virus from the mouse to chick-embryo was established. This fact suggests that the virus stays in the organs of the infected mouse for a long time and its multiplication and diminution are being repeated.