In our laboratory, many studies on the infectious hepatitis in Okayama Prefecture, particularly on its pathogenic agent have been carried out. The infection tests on animals, of which some parts were reported, were already finished. In the present studies the author tried the serological studies, which would be the key to determine the nature of isolated virus. In these days of having no definite method for the serological diagnosis, it is not easy to appraise the value of complement fixation tests which the author tried. From the following results, however, the author was convinced that this reaction was also of practical value: 1) The antigens were prepared by Larin's or Casals's method from the liver pool of mice infected with the hepatitis virus by the abdominal inoculation. From the results of antigen titer measurement, it was certified that these antigens were worthy of practical use. 2) The complement fixation tests with the sera of convalescents gave significant results. in which even the case of positive reaction of 1. 32 of antigen titer was observed. With the sera of so-called chronic hepatitis, however, the complement fixation titer was generally low and not over 1:8.