1. 1. The channel catfish, Ictalurus punctatus, was immunized with 2,4-dinitrophenol-protein conjugates and found to produce two different molecular weight species of antibodies when tested with DNP coupled to human erythrocytes. 2. 2. The higher molecular weight antibody was isolated and found to have a molecular weight of 610,000 with a sedimentation coefficient of 14 S. 3. 3. The carbohydrate composition differed from that of human macroglobulins, having a mannose/galactose ratio of approximately one and a total carbohydrate content of 5.5%. 4. 4. Complete reduction and alkylation of this macroglobulin, followed by gel filtration in 5 M guanidine hydrochloride, resulted in the separation of the heavy (H) and light (L) polypeptide chains. These chains had molecular weights of 58,000±2500 and 22,500±2500, respectively, as determined by column chromatography. 5. 5. These data are most consistent with a tetrameric macroglobulin molecule. © 1973.