The author studied the presence and the natures of the lethal toxin of the virus isolated from the patients of hepatitis in Okayama prefecture. The results were as follows: 1) The toxin of hepatitis virus has nearly the same natures as those reported about other sorts of virus and rickettsia, and its presence is proved by the death occurring in a short time after the inoculation of the embryonated egg-culture into the caudal vein of the mouse. 2) Some difference of the toxicity of this lethal toxin was observed among the strains of virus. The toxicity generally tended to fall by the successive cultivation of the virus in embryonated eggs. When the newly isolated virus is used, the toxicity varies with the quantity of the virus and is strong when the virus multiplies very well and the inoculated embryos show the high mortality. 3) The fact that the lethal toxin comes from the virus itself is proved by the results of neutralization test and others, though definite results are not yet obtained in the neutralization test. 4) The naturs ot the toxin are common to all virus strains, and, judged from the general natures, this lethal toxin is very much alike to the virus itself. though its essential nature is still in question and needs further studies. It is considered, however, that the lethal toxin controls the natures of the virus and is greatly related with its hepatitis-causing nature.