A total of 20 hybridoma clones secreting monoclonal antibodies (MAbs) were established between mouse myeloma cells and spleen cells from mice immunized with red sea bream iridovirus (RSIV) isolated from diseased red sea bream Pagrus major. These MAbs reacted with RSIV-infected BF-2 cells but not with uninfected BF-2 cells in immunofluorescence assays. With these MAbs, 2 classes of RSIV specific polypeptides (230/180 kDa, 20/16 kDa) were characterized by immunoprecipitation and SDS-PAGE. All 20 MAbs reacted with BF-2 cells infected not only with RSIV isolates from various geographic areas, but also with sea bass iridovirus (SBIV) isolated from diseased sea bass Lateolabrax sp. Twelve of the 20 MAbs reacted positively with BF-2 cells infected with Japanese parrot fish iridovirus (JPIV) from diseased Japanese parrot fish Oplegnathus fasciatus. None of the MAbs reacted with EK-1 cells infected with icosahedral cytoplasmic deoxyribovirus (ICD virus) isolated from diseased Japanese eel Anguilla japonica, or with FHM cells infected with Frog virus 3 (FV3) belonging to the family Iridoviridae.