Segment A of the genome of infectious bursal disease virus(IBDV) encodes structure protein VP2 and VP3 and protease VP4. In this study a polyprotein gene of IBDV was inserted into a Bombyx mori baculovirus transfer vector pAcHLT--C and contransfected into BmN cells with linear genome DNA of virus Bm-BacPAK6. Dot hybridization suggested that the segment A of the virus genome was inserted in the genome of Bm-BacPAK6. The silkworm of fifth instars were infected by the recombinant virus and the immunogenicity of the infected larvae's blood were examined with ELISA, SDS-PAGE and Western blotting. It appears that the recombinant polyprotein has the property of immunoreactivity and the expression in larvae reached the pick 5-day post infection.