The single radial haemolysis test for the serological diagnosis of suspected dengue and Japanese encephalitis virus infections uses crude virus antigens with a haemagglutinin titre of 1:320 or 1:640. The results, which may be read 3 hours after the addition of a patient's serum, showed a general agreement between this test and haemagglutination-inhibition tests in the number of case diagnoses that were confirmed. The antibody responses of individual patients shown by the two tests, however, were different, which suggests that the two tests may not be measuring the same antibody. The single radial haemolysis test can distinguish between dengue and Japanese encephalitis viruses using specific mouse hyperimmune sera. Tests on a limited number of sera from Japanese encephalitis patients also showed no cross-reactions with dengue virus antigens in those cases having a low-titred but significant fourfold antibody rise to Japanese encephalitis antigen.