The comparative studies on the viruses isolated from different materials and by different methods were carried out sero-immunologically. The findings are summarized as follows: 1) Using the viruses isolated from different materials and by different methods, the "Absättigunsversuch" was tried. Though the “Absättigungsversuch” was somewhat different from the original wildführ's method, unacceptable results were sometimes obtained when the sera of hepatitis patients were used. In the mouse the neutraization test was well established between the virus and the immune serum of high titer, and this was proved by the nonappearance of the pathological changes in the mouse. 2) For the inactivation of the virus, ultra-violet irradiation was more simple and certain than the addition of marzonine. It was proved. moreover, that thus inactivated virus still preserved the antigenicity. 3) In the infection-protecting test, the immunization by the inactivated virus protected the attack not only by the same living strain but also by the different strains. 4) These above-mentioned results suggest that the viruses isolated from different materials and by different methods come all from the hepatitis patients and have a pathogenic significance.