In order to reinvestigate the natures of hepatitis virus in detail, the author studied the general natures of hepatitis virus in comparison with ectromelia virus. The results are summarized as follows: 1) Though hepatitis virus and ectromelia virus are of great similarity in their various natures, collective consideration of the heat-resistance, the drug-resistance. the hemoagglutination reaction etc. makes possible the differentiation between these two sorts of viruses. 2) In the serial passage in mice, though the mixed infection of ectromelia virus should be considered, the discrimination between these two sorts of viruses is possible by observation of infectious aspects and general natures.