A total of 27 clones of hybridoma cells secreting monoclonal antibodies (MAbs) against VDV, the causative agent of viral deformity in yellowtail Seriola quinqueradiata, were generated. These MAbs reacted to VDV-infected CHSE-214 cells but not to uninfected CHSE-214 cells by immunofluorescence test. By using these MAbs, 2 classes of polypeptides (VP2, VP3) were characterized by radio-immunoprecipitation test or Western blot. Thirteen MAbs among 24 MAbs reactive to VP2 polypeptide neutralized virus infectivity but the other 3 MAbs specific to VP3 did not neutralize the infectivity. The reaction patterns of the MAbs obtained to the 3 reference strains (VR-299, Sp and Ab) of IPNV were variable. However, neutralization epitopes were conserved on VP2 of both VDV and IPNV. Six isolates of birnaviruses from marine cultured fishes were studied for reactivities of the MAbs by immunofluorescence test. The MAbs showed same reaction patterns to an isolate (YAV-1) from yellowtail and an isolate (H-1) from Japanese flounder Paralichthys olivaceus as those of VDV by immunofluorescence test. However, some MAbs did not react to the other 4 birnavirus isolates from Japanese flounder.