The saturated ammonium sulfate method was used to crude-extract the IgM of channel catfish, then purifying the IgM using sulfur resin affinity chromatography, followed by immunizing the rabbit using the purified IgM and detecting the antibody titer of the rabbit anti-IgM serum using the enzymelinked immunosorbent assay (ELISA). We prepared the recombinant protein pSCPI based on the method used in Chapter 2, vaccinating the healthy catfish with the concentration of the antigen protein 1 μg/g fish, 2 μg/g and 3 μg/g fish, respectively and with PBS as a control. We detected the antibody level of the specific antibody produced by the vaccinated catfish at 21,28,35,42 and 49 days post-vaccination, and challenged with the S.iniae DGX07 at 4 weeks post-vaccination. The results showed that: the purified IgM heavy chain and light chain by using sulfur resin affinity chromatography were about 70 ku and 27 ku, separately. And the concentration of the IgM was up to 6.4 mg/mL, the antibody titer of the rabbit anti-IgM serum was 1∶51 200. Also we observed that the antibody level began to increase at 3 weeks post-vaccination and with the highest antibody level occurring at 4-week p.v., but it began to decline at 5-week p.v.. The antibody level of the antigen concentration with 3μg/g fish-vaccinated fish was highest at each examined time point, which had no significant difference compared with the 2 μg/g fish-vaccinated fish. In 1 μg/g, 2 μg/g and 3 μg/g fish-vaccinated fish, S. iniae DGX07 challenge caused accumulated mortality rates of 47.5%, 45% and 47.5% respectively, which corresponded to RPS rates of 40.63%, 43.75% and 40.63% respectively with PBS as a control. For all vaccination trials, examination of moribund fish indicated that S. iniae was the only type of bacterial strains isolated from S. iniae-challenged fish, suggesting that mortalities were caused by the challenging bacteria. All these results indicated that the recombinant protein pSCPI was effective and had a better immunoprotectivity, which could be used as a candidate subunit vaccine against S. iniae.