The influences of P(32) upon the mice infected with Japanese B Encephalitis were investigated. On the course of this investigation, the effects of P(32) upon the formation of nuclear inclusion-body was also histologically determined on the brains of infected mice. With the mice which had been intracerebrally injected with the virus at the concentration of about LD(50) the delay of infection and prolonged survival time were usually observed when therapeutic doses of P(32) were intraperitoneally given. This was also the case when the virus had been inoculated intraperitoneally or intravenously. In all these cases, the formation of nuclear inclusion-body was always more distinct than that of the control references.