The indirect method of immunofluorescence was used to titrate the antibody (Fluorescent antibody, FA) response in patients of Japanese encephalitis, after confirming that they had Japanese encephalitis by complement fixation test and hemagglutination inhibition test.The fluorescent antibody response patterns of 53 patients were studied. FA was found to appear early in the course of the disease, and tended to increase through the second or third post-onset week, although a few cases had maximal FA titer in the early stage of infection.The criterion for the serological diagnosis of Japanese encephalitis using indirect FA test was tentatively established as follow: more than 4-fold conversion should be observed between acute and convalescent specimens the latter titer being ≥1:80, or the serum at any stage should have a titer ≥1:160.According to the above criterion, 52 of 53 patients were positively diagnosed by the FA test. On the other hand, a diagnosis of Japanese encephalitis was made 51 cases out of 53 using HI test, and 36 cases out of 51 using CF test.Further analysis of the using various serological methods showed positive diagnosis were obtaind by HI test in 50% of patients, by FA test 30%, and by CF test 10% during the first 10 days of illness, by HI test was 90%, by FA test 75%, by CF test 30% during the first 20 days of illness.It is to be noted that FA test provide us a rapid reliable serological method for diagnosis. It takes about 200 minutes to examine one batch of 10 specimens with this test.Maximal FA titer obtained with case of inapparent Japanese encephalitis virus infection did not exceed 1:80, and those of normal cases 1:40 regardless their history of vaccination.