On the basis of the construction of an expressed vector which expresses a bivalent out membrane protein (OMP) of Aeromonas hydrophila and Edwardsiella tarda from diseased eels,optimum expression conditions,including different IPTG concentration,temperature,time and bacteria concentration of the recombinant expression vector were studied.The results indicated that different IPTG concentration,temperature,time and bacteria concentration had no significant influence on the expression of the recombinant protein.A bivalent OMP was expressed by using 0.25 mmol/L IPTG to induce the tansformant (E.coli BL21) overnight at 16℃ in a suitable bacteria concentration (D600nm=0.8).The bivalent OMP with a molecular weight of 87.1 ku was then purified and obtained by Nickel Chelating Affinity Chromatography with KTApurifier-100 protein purification system.After renaturated by dialysis,the purified protein was injected to American eels (Anguilla rostrata) to study its immunogenicity.The results indicated that the titers of serum antibody in eels at 28 d after immunization,was significantly (P＜0.05) enhanced in groups injected with different doses of the recombinant protein compare with the PBS injection group,indicating good immunogenicity of the recombinant protein.