Using the hepatitis virus isolated from hepatitis patient, the author cultured the virus by serial passage in chorio-allantoic cavity of chick embryo, collected it in purified form by means of ultracentrifuge and observed the resistance to heat that was supposed to be a important feature of the virus. For the infection of the virus it does not render the lethal effect on mouse, only pathological findings have the significance for identification of the infection. On account of this, the author adopted pathological findings on the liver of the mouse, that was inoculated the heat-treated purified virus, as to determin the resistance to heat. The following results were obtained. 1) From the results of repeated test that the heat resistance of purified virus was examined, it could be concluded that the heating at 55°C for 30 min. did not affect its activity at all; but the heating at 60°-65°C for 30 min. showed serious effect on the activity, however, whole virus could not be inactivated by this treatment; and the heating at 70°-75°C for 30 min. inactivated the virus completely. 2) The result of the inactivation test described above, in that the complete inactivation of the virus was achieved by heating at 70°-75°C, was confirmed further by the serial passage of the virus. However, the resistance of the virus was supposedly varied to some extent by the heating method and the pH of medium at heating.