1) The results of mouse neutralisation tests with intraspinal inoculation technic were compared with those obtained by HeLa cell tissue culture neutralisation method, using three_ types of poliomyelitis virus vs. diluted sera (1:10) from 12 typical cases of paralytic poliomyelitis. Only one serum specimen gave a clear cut positive by tissue culture neutralisation test, but negative results by mouse neutralisation test, and the remainder showed complete agreement between the results of both methods.This preliminary study indicated that the neutralisation tests in mice with mouse-adapted polioviruses were utilized effectivly in circumstances which tissue culture neutralisation test could not be done.2) In 1955, 111 sera were collected from inhabitants of Okayama area. The percentage positive of neutralizing antibodies examined in mice with these diluted sera (1:10) against 100 PD50 of type I and II, and 20 PD50 of type III were followed: in placenta cord serum, against type I 83.3%, against type II 66.7%, but type III 33.3%; thereafter these percentage decrease rapidly, reached the lowest level in 6 months to 1 year group. Then the positive percentage against type I and II sharply rose up from 2-4 years group and reached the adult level in 10-14 years group against type I and in 5-9 years group against type II. But during 2-19 years old, the positive percentage against type III were still remained about 20-29%. These pattern of antibody-level illustrated that the distribution of type I and II of poliomyelitis virus in Okayama were resemble to those of Tokyo and Osaka, but the one of type III showed endemic nature and its infections seemed not to be prevalent in our area.