The fluorocarbon deproteinization technique was demonstrated to be a useful method for the purification of Japanese B encephalitis (JE) virus from infected mouse brain. Electoron microscopic observation on purified virus particles by this method schowed numerous particles which were presumed to be JE virus. Electoron microscopic observation on the surface of the chick red cells which were precipitated with purified virus (hemagglutination) displayed similar particles. The JE particles were hexogonal in schape, and composed of an outer membranes and a centrally located nucleoid-like structure. The size of the particle measured 30 to 40 mμ in diameter. A substance which cause the hemagglutination was conservable to be the virus particle itself.