The principle of baculovirus expression system is that substitute exogenous gene for polyhedrin gene (poly),and the recombinant baculoviruses lack the ability to infect insect larvae by oral inoculation.In order to deal with this problem,we cloned the poly gene with its promoter of Bombyx mori nucleopolyhedrovirus (BmNPV) into pigA3GFP vector,and transported it into BmN cells by lipofectamine with helper vector pHA3PIG.After several rounds of selecting,the transgenic BmN cells were obtained.Then the poly gene negative viruses,BmPAK6 (Lac Z) and BmGFP (GFP),infected the transgenic BmN cells.Subsequently,the viruses (BmPAK6 and BmGFP) from infected cells were used to orally inoculated the 5th instar larvae of Bombyx mori (B.mori),respectively.The results showed that B.mori larvae could be infected per os with the recombinant baculoviruses BmPAK6 and BmGFP,respectively.These results suggest that the products of poly gene expressed in the transgenic BmN cells could raise the pathogenicity of the recombinant virus in orally infected B.mori arvae.