By the cultivation in fertile hen's eggs, a kind of virus which could be regarded as the agent of hepatitis was isolated from the hepatitis patients in Okayama Prefectur. Under various conditions, the author carried out the experiments to study the heat-resistance of the isolated virus. The estblishment of infection was judged by the pathological changes of inoculated mouse liver. The results were as follows: 1) The isolated virus was highly resistant to heat, but was inactivated by heating at 75°C for 30 minutes. This was a very characteristic nature of the isolated virus to differentiate it from other simillar viruses. 2) In respect to the nature of heat-resistance, this virus was inferred to be very similar to the one described by Havens in his human body experiment.