As for the immunological natures, particularly for the neutralization test and the "Absaettigungsversuch", the isolated hepatitis virus was compared with analogous viruses. The results are as follows: 1) Both of the neutralization test and the "Absaettigungsversuch" of ectromelia virus are ef practical use ot least to a limitted degree. The “Absaettigungsversuch” is, however, of less use than the neutralization test, and is inferred to give a definite result only in combination with the neutralization test. Consequently, combined employment of the both tests is desirable. 2) Hepatitis antisera showed a high neutralization with hepatitis virus, and not with the other viruses. The result of this neutralization test was determined by observation of pathological findings. 3) In short, the results of these immunological tests showed that the viruses of infectious hepatitis, ectromelia and "Izumi fever" were clearly different to each other, and distinct discrimination was possible among them.