Immune sera were obtained from four rainbow trout that had survived natural infection by infectious haematopoietic necrosis virus (IHNV), and five monoclonal antibodies (MAbs) were prepared against a Korean isolate of IHNV, IHNV-PRT. These immune sera and MAbs were characterized in terms of IHNV-neutralizing properties and reactivity in Western blots with the viral proteins of IHNV-PRT. All five MAbs and four immune sera neutralized IHNV-PRT to various extents. Antibodies in these immune sera recognized two structural proteins of IHNV, G and M1, and one protein with a molecular mass of 90 kDa. Of the five MAbs, three (AB9, AF6 and AG6) recognized the IHNV G protein, and the other two (AB7 and BC2) recognized the 90 kDa protein. The 90 kDa protein was found to be a cellular protein constitutively expressed at low levels in fish cells and expression of this protein was augmented by infection with IHNV and heat shock. MAbs specific to four stress proteins, hsp60, hsp70, hsp90 and grp94, failed to bind to this 90 kDa protein. MAbs AB9 and AB7 reacted fairly broadly with six different IHNV strains. Together, these results indicate that (1) two IHNV proteins, G and M1, and a 90 kDa cellular protein are immunogenic, (2) G and the 90 kDa proteins contain neutralizing epitopes, and (3) the epitopes recognized by MAbs AB9 and AB7 are conserved among the six different IHNV strains.