Interleukin 21 (IL-21) is a novel typeⅠcytokine that is significantly homologous to IL-2, IL-4 and IL-15. Its receptor complex containsγc chain which is also a component of receptors for IL-2, IL-4, IL-7, IL-9 and IL-15, so there may be overlapping or relevancies in their biological functions. IL-21 is capable of co-stimulating mature T cells, B cells, NK cells, and of stimulating CD16 expression on the surface of NK cells to induce ADCC in innate immune response. It can also strengthen the anti-tumor effect of the cellular immunity, especially via enhancing the activities of NK and antigen specific CTL cells. Thus, IL-21 is a potential useful therapeutic molecule for immunotherapy of malignancies, by eliciting innate and adaptive anti-tumor immune responses in tumor-bearing hosts. In order to study the biological functions of IL-21, we constructed a mIL-21 prokaryotic expression plasmid and expressed the recombinant mIL-21 protein in E. coli in present study. The recombinant plasmid pET28a/mIL-21 with a carboxyl terminal His-tag was subcloned from the pcDNA3.1/mIL-21 and expressed in E. coli. The induced protein was detected by SDS-PAGE, and identified by Western-blot assay with anti-mIL-21 antibody. The recombinant protein was purified via Ni+ affinity chromatography, and renatured with GSH/GSSG system. Our mouse T cell proliferation experiment showed that the recombinant mIL-21 protein could enhance the mouse T cell proliferation either by itself alone or in the presence of Con A. Its receptor complex contains γc chain which is also a component of receptors for IL-2, IL-4, IL. Interleukin 21 (IL-21) is a novel type I cytokine that is significantly homologous to IL-2, IL- IL-21 is capable of co-stimulating mature T cells, B cells, NK cells, and of stimulating CD16 expression on the surface of NK cells to induce ADCC in innate immune response. It can also strengthen the anti-tumor effect of the cellular immunity, especially via enhancing activities of NK and antigen specific CTL cells. Thus, IL- immunotherapy of malignancies, by eliciting innate and adaptive anti-tumor immune responses in tumor-bearing hosts. In order to study the biological functions of IL-21, we constructed a mIL-21 prokaryotic expression plasmid and expressed the recombinant mIL-21 protein in E. coli in present study. The recombinant plasmi dpET28a / mIL-21 with a carboxyl terminal His-tag was subcloned from the pcDNA3.1 / mIL-21 and expressed in E. coli. The induced protein was detected by SDS-PAGE, and identified by Western-blot assay with anti The recombinant protein was purified via Ni + affinity chromatography, and renatured with GSH / GSSG system. Our mouse T cell proliferation experiment showed that the recombinant mIL-21 protein could enhance the mouse T cell proliferation either by itself alone or in the presence of Con A.