The author purified the infectious hepatitis virus by the ultra-centrifugation, and studied its serologic characters. The results are briefly summarized as follows: 1) A high titer immune serum could be obtained by many times immunization with the purified virus; the complement fixation index was I; 640 to I: 1280 both in Ishihara and Ogawa strains. 2) The complement fixation reaction with the purified virus gave good results to the convalescent sera diagnosed as acute infectious hepatitis, but the reaction to the sera of the Chronic type of infectious hepatitis was not definite. 3) These results of the complement fixation test can be considered the clear evidence that the isolated virus is the one from the patients of infectious hepatitis. 4) In the neutralization test or Absaettigungsversuch with the serum immunized by the purified virus, a significant result can be obtained with the serum of high complement fixation titer. With the serum of low complement fixation titer, however, the result was often equivocal; the performance of this reaction should be very careful.