In order to study the possibility of successive transmission and the determination of infection titer of hepatitis virus in fowls, the author used six strains of virus, Ishihara Kanamitsu, Noda, Ogawa, Morimoto, and Aomori stains, which were isolated from the patient materials by the successive transmission through mice and embryonated eggs in our laboratory. As the criteria of infection, the author took the hepatitis-specific pathologic changes of the liver and lung. Of the fowls tested, infection was the severest in Chloris sinica minor T. & S, and became milder in the following order: Chloris sinica minor T. & S. Uroloncha domestica, Serinus canaria, Passer montanus saturatus Stejeneger, Emberiza rustica latifascica Portenko, Pyrrhula pyrrhula griseiventris Lafresnaye, Emberiza cicoides ciopsis Bonaparte of Passeres and Fringillidæ, and zosterops palpebrosa japonicaT. & S. of Zosteropidæ. In each fowl, the histological changes became severer with the successive transmissions, and seemed to be fixed at about the fifth transmission. As to the determination of infection titer, the most remarkable pathologic changes were observed around 10(-9)× in Serinus canaria and 10(-10)× in Uroloncha domestica. The tests of reversion of virus from fowls to mice were positive in all cases.