Abstract: Serum immunoglobulin (Ig) from channel catfish (Ictalurus punctatus) immunized with Aeromonas so-bria was purified with ammonium sulfate precipitation followed by column chromatography.The results showed that serum Ig was mainly precipitated at 35%-55% ammonium sulfate saturation.Further purification by Sepharose-4B gel filtration and DEAE-52 anion exchange chromatography showed that the Ig from both of the columns all presented in the first protein peak.SDS-PAGE demonstrated that serum Ig was more pure after each step of purification.Structure analysis of the catfish serum Ig was performed by Western blot under denatured and reduced, denatured but non-reduced, non-denatured and non-reduced conditions using monoclonal and polyclonal antibodies as probes.The results showed that the molecular weight of the heavy chain was 72 kD, and the three distinct light chains were 21 kD, 23.5 kD, 26 kD respectively.The results suggested the heterogeneity were presented in channel catfish light chains.The serum Ig only had a molecular weight about 870 kD under non-reduced and non-denatured conditions.