The purification of serum immunoglobulin (Ig) of European eel（Anguilla anguilla）was carried out by affinity chromatography. The structure and the antigenicity of eel’s serum Ig were analyzed by electrophoresis and Western-blotting. The results showed that after separating by the affinity column, the amount of eluted protein of serum Ig was showed as a sharp curve on plotting paper. And the protein curve was highly correlated with the antigenic activity curve when analyzing the sample by ELISA. SDSPAGE dem onstrated that the heavy chain of the serum Ig was 68 kD in size, there were three light chains with the molecular weight of 21 kD, 23 kD and 26 kD respectively. Under nonreduced and nondenatured conditions, the serum Ig appeared as two protein bands with the molecular weight 790 kD and 350 kD, and showed 790 kD, 593 kD and 350 kD three protein bands under denatured and nonreduced conditions. Western-blotting revealed that the rabbit antiserum could recognize various polymers, heavy chain of the serum Ig, but could not react with the light chains. In combination with the above results, the natural form of the serum Ig could be presented as tetramer and dimer, which was different from other serum Ig of teleost. The disulfide linkage between the heavy chains of the Ig was incomplete. Under the reaction of the SDS the Ig had come apart into various smaller polymers. There were three distinct light chains found in the Ig of European eel for the first time.