Putting great stress on the infection by the mouth, the author studied the problem of infection routes of the infectious hepatitis virus. The virus isolated from the patients of infectious hepatitis was used throughout this work. The establishment of infection was judged by the pathological changes in the inoculated mice, which were employed as the test animal. The results were as follows: 1) From the results of the experiments on mice, it was presumed that no remarkable difference of virulence was present among the isolated virus strains, 2) Of all the routes tested, the infection was most easily established by the administration by the mouth, and this caused also the remarkable pathological changes in animals. Consequently the mouth was considered to be the most favorable portal for the infection.