Japanese encephalitis (JE) viruses are grouped into four genotypes. Although currently available vaccines are derived only from viruses in genotype III, vaccines are known to protect against naturally occurring strains. Studies were undertaken to assess the suitability of a freeze-dried pool of human anti-JE plasma, collected from recipients of Biken (Nakayama-NIH) killed vaccine, to serve as an International Standard for antibodies to JE virus. Five participants in five countries submitted data from 11 assays on the candidate International Standard and seven coded samples including sera from recipients of vaccines containing a range of virus strains. The results of the study indicated that the 50% plaque reduction neutralization test (PRNT(50)titres) obtained for serum from recipients of killed vaccines, including the candidate standard, vary depending on the virus strain used in the neutralization tests, namely higher PRNT(50)titres were obtained when the challenge virus was homologous to the vaccine strain compared to use of a heterologous virus. Potencies expressed relative to the candidate standard are therefore affected by the strain of virus used in assays and the use of a standard would therefore not facilitate direct comparison of data from laboratories that have used different challenge strains.