Abstract: Objective To express the gene encoding surface antigen one(Sao)in highly virulent strains of Streptococcus suis serotype 2(S.suis 2)and to prepare the polyclonal antibodies against Sao to assess the immune characteristics of the anti-Sao polyclonal antibodies.Methods The target gene sao was amplified by PCR using the genomic template of 05ZYH33.Sao gene was subsequently inserted into pEASY-T1-simple vector and subcloned into expression vector pET28a.The prokaryotic expression plasmid pET28a-sao was transformed into E.coli BL21 after the identification with double digestion of BamH I and Sal I and DNA sequencing.With the induced expression by isopropyl β-D-1-thiogalactopyrannoside(IPTG),the recombinant protein Sao was analyzed by sodium dodecyl sulfate polyacrylamide gel electrophresis(SDS-PAGE).The Sao protein was purified with His-Binding-resin.The titer and specificity of the polyclonal antibodies were determined by enzyme-linked immunosorbent assay(ELISA)and Western blot.Bactericidal assay was used to identify the bactericidal activity of rabbit polyclonal antibodies against Sao.Results The expression vector pET28a-sao was successfully constructed.The fused gene was highly-expressed in E.coli BL21.ELISA detection results indicated that the titer of polyclonal antibodies to Sao was about 1:102400.Western blot results showed that the anti-Sao polyclonal antibodies could bind to recombinant Sao protein specifically.The polyclonal antibodies against the recombinant Sao protein could promote bactericidal ability of human whole-blood.Conclusion The anti-Sao polyclonal antibodies are of good specificity and high titer and could promote bactericidal ability of human whole-blood.