Following the studies in part I and II, the author studied the establishment of infection by the oral administration of virus, the mechanism of infection, and viremia. These were investigated chiefly by the observation of pathological changes of the tested mice. The results were as follows: 1) The investigation of the pathological changes and the distribution of virus in the animal body revealed that the fixation and multiplication of the virus occurred in the liver, and, in parallel with these, the viremia went and reached the maximum around the 14th day after the injection of virus into the stomach. 2) As for the essential mechanism of the alimentary infection, the previously infered results were obtained. In case of the chronic infectious course, however, there remained some questionable points as to the mechanism of the establishment of infection and viremia.