Use of the single-radial-haemolysis (SRH) technique for the diagnosis of flavivirus infections is described. A large number of paired and single convalescent serum samples collected from cases of encephalitis during two major outbreaks in Kolar district of Karnataka State in India during 1977 and 1979 were tested by this technique. The results were compared with those obtained in the haemagglutination inhibition (HI) test in all cases, and the complement fixation (CF) and neutralization tests in some cases. Japanese encephalitis virus was shown by the SRH test to be the major etiologic agent responsible for both epidemics. This was corroborated by the HI, CF and neutralization test results. The single-radial-haemolysis test was found to be simpler and more specific and sensitive than the haemagglutination inhibition test.