Envelope glycoprotein of avian leukosis virus Subgroup J (ALV-J) determines the host range of virus infection and cross-neutralization patterns. The truncated envelope genes of avian lecukosis virus subgroup J (ALV-J) were amplified by PCR and cloned them into pGEX-5X-3 vector for expressing envGST-fusion protein. Western blot analysis results showed that the products of truncated env gene expressed in Escherichia coli could reacted with G2, JE9 and I45 monoclonal antibodies (Mabs) specific to envelope protein of ALV-J. Using different Mabs to map the epitopes in the expressed truncated gp85 GST fusion protein, the results showed that Mab G2 and JE9 antibodies recognizing epitope in gp85 was localized between amino acid 65-155. Mab I45 reacted with the epitope at the location of amino acid 156-233. It indicated that the specificity of subgroup J virus is determined by the gp85 peptide since GST-gp85 protein expressed in Escherichia coli is not glycosylated.