Serological studies before and after Japanese encephalitis (JE) vaccination were made on subjects of various ethnic groups with or without dengue and/or JE antibodies. In these subjects dengue and JE hemagglutination-inhibition (HI) and neutralization (N) antibodies were measured. Seroconversion to JE was observed in about 45% of the subjects by the HI test and 63% by the N test, whereas seroconversion to dengue with only 4-fold increase in titer after vaccination was detected in 14% by the HI test and in a single case (3%) by the N test. The presence of JE and/or dengue antibody in the prevaccination sera did not significantly influence the seroconversion rates of JE-HI and N-antibodies after vaccination. Moreover the seroconversion rate did not seem to vary markedly in different ethnic groups, although tendencies for subjects from Southeast Asia to have dengue antibodies and Japanese people to have JE antibodies were demonstrated. The presence of dengue N-antibodies in the prevaccination sera did not significantly influence the geometrical mean titer of postvaccination JE-N antibody. Thus the antibody response to JE vaccination was fairly specific to JE, and no marked secondary response was observed.