The polypeptide product of gene 6 of the IDIR strain of group B rotavirus was synthesized by means of a baculovirus expression system in order to confirm the coding assignment of the gene and to develop reagents broadly reactive with heterologous strains of Group B rotaviruses (GBR). Earlier experiments indicated that IDIR virus gene 6 encoded the group-specific, major inner capsid protein, but direct confirmation of this coding assignment was not previously reported. The expression of IDIR virus gene 6 from baculovirus recombinants resulted in production of a protein with an apparent molecular weight equivalent to that deduced from the gene sequence (44 kDa). In addition, larger recombinant proteins were also observed, and these appeared to be consistent in size with oligomers of the primary VP6 product. The expressed protein reacted with antibody directed against the IDIR agent and other strains of GBR, but no reaction was observed with antibody directed against group A rotavirus. Serologic reactivity was also observed between the gene 6 product and a monoclonal antibody directed against the GBR group-specific antigen. Antibody directed against the recombinant gene 6 product specifically reacted with the IDIR virus major inner capsid protein in an immunoblot format. These experiments conclusively demonstrated that IDIR virus gene 6 encoded the major inner capsid protein and confirmed the presence of group B-specific antigenic epitopes on the protein.