MURAKAMI and his collaborator had reported already the properties of the hepatitis virus isolated from hepatitis patient in detail, and they pointed out some similarity of the virus with the IZUMI fever virus. The author obtained both viruses in purified form by means of ultracentrifuge, and carried out further investigation on the virological properties of these viruses. The following results were obtained. 1) It was confirmed that both viruses could be obtainable easily and efficiently in purified form by means of ultracentrifuge, and showed powerful pathogenecity to mouse capable of inducing the typical pathological changes by infection; however, the infection of either virus did not render the lethal effect and was detectable as latent infection in that only the pathological findings were informative. And by the electronmicroscopical study, both viruses were observed in minute round or oval form whose diameter was 50-70 mμ. 2) From the viewpoint of the virological properties, both viruses were much different in the resistance to heat and disinfectant: the purified hepatitis virus could stand aginst heating at 70°-75°C and against the action of disinfectant, 0.2% formalin or 0.01% marzonin, for less than 3 weeks; on the other hand the IZUMI fever virus was inactivated by heating 55°-60°C for 30 min. or by action of the disinfectant for 2 weeks. 3) By the irradiation of ultraviolet rays, it is needed to inactivate the purified hepatitis virus for 40 min. irradiation, but in the case of IZUMI fever virus it was completely inactivated by only 5-20 min. irradiation.