Immunization of specimens of the European eel with a hapten‐carrier conjugate resulted in a significant rise in anti‐hapten titres. Antigen‐specific immunoglobulin was purified on a matrix onto which the hapten‐carrier conjugate had been immobilized. Rabbit antisera raised against the product of the affinity purification recognized two molecular moieties. The first was eel immunoglobulin as inferred by the polypeptide composition. i.e. disulphide‐linked heavy and light chains of 72 kDa and 25 kDa respectively. The second was a 110‐kDa protein. A 800‐kDa and a 400‐kDa molecular form of eel immunoglobulin were disclosed by a combination of gel filtration and immunoelectrophoretic detection. The latter was the more abundant serum form. The 110‐kDa protein was found non‐covalently associated with the 400‐kDa immunoglobulin. Both molecular forms of immunoglobulin from immune sera exhibited antigenic specificity and reactivity.