The total of 568 human sera (408 cases in male and 160 cases in female) were obtained from the out-patients in National Seoul Hospital on June to December, 1979. The hemagglutination inhibition antibody titer of these human sera against Japanese encephalitis virus was examined. The following results were obtained. 1. The positive rate of HI antibody was confirmed of the 568 cases and among these positive groups, the positive cases of more than 1: 160 dilution were only 50 cases (8.8%). 2. The rate of HI antibody titer in the sexual group showed no significant differences; 62.2% of the 270 cases in male and 65.6% of the 105 cases in female showed as a positive. 3. The positive rate showed widely in each age group, but the 30 to 50 old age group showed higher (69%) than others.