Serotyping of human rotavirus in the Tokyo area was conducted from 1990 to 1993 by enzyme immunoassay with monoclonal antibodies (EIA-MAbs) against VP7 and by reverse transcription and polymerase chain reaction (RT-PCR) amplification of the VP4 and VP7 genes. The results by EIA-MAbs were very similar to those obtained by RT-PCR. Evidence of intraserotypic variations was suggested because strains of undetermined serotypes were detected by either EIA-MAb or RT-PCR. This kind of study is required for vaccine development.