In order to discriminate the isolated infectious hepatitis virus from mouse hepatitis virus (Buscher strain), of which both are very similar in their various natures, the author carried out various experiments and obtained the following results: 1) Only the suckling mouse is susceptible to mouse hepatitis virus and succumbs to its infection, whereas it is rarely killed by the isolated hepatitis virus. 2) The chick embryo is little susceptible to mouse hepatitis virus, but is highly susceptible to hepatitis virus and is killed in a few days. 3) Whichever of the suckling mouse and chick embryo may be used as the test animals, and whichsoever of both viruses as the interfering or challenging virus, no interfering phenomenon can be observed between mouse hepatitis virus and the isolated infectious hepatitis virus. 4) In the immunological test, "Absaettigungsversuch", no relation can be observed between these two viruses. All of these results suggest that mouse hepatitis virus and the isolated infectious hepatitis virus are clearly different to each other.