This report describes an investigation of the antigenic localization of Japanese B encephalitis virus in the brain of infected mice that were inoculated intracerebrally with seed virus. 24 hrs. after inoculation, diffuse specific fluoreacence were observed in large cells of the cereberal cortex. In those cells, viral antigen were predominantly present arround the nucleus. In some cells, they spread into the nucleus from the perinuclear region and in others, into the cytoplasm. In case of the brain of mice that manifested typical cereberal signs, fine fluorescent granules were noticed in nerve cells and some glia cell of the cereberum. The majority of the viral antigen appeared to be present in the nucleus. The cells having the viral antigen were most numerous in the region of the substantia nigra, thalamus and basal ganglions. Those cells were also numerous in the cortex and cerebellar cortex. There were no specific fluorescein in the chorioidea and meninges.