AIM: Expression and analysis of recombinant chicken IL-18 in Pichia pastoris. METHODS: Chicken IL-18 mature peptide gene was amplified from the recombinant plasmid pMD18-T-ChIL-18 by PCR, and was subcloned into Pichia pastoris expression vector pPICZalphaA to construct the recombinant plasmid pPICZalphaA-ChIL-18. After identified by restriction enzymes digestion analysis, PCR and DNA sequencing, the recombinant plasmid was transformed into Pichia pastoris X-33.Then choosing the multi-copy recombinant strains to be induced for expression.Then the bioactivity of rchIL-18 was analysed by Western blot, ELISA and MTT after purified by Sephadex G-100 column. RESULTS: The chicken IL-18 with the immunogenicity was secreted by Pichia pastoris. It could induce T lymphocytes proliferation and secreting IFN-gamma in vitro. CONCLUSION: The chicken IL-18 with obvious biological activity is secreted by Pichia pastoris X-33.