Macroglobulins isolated from fresh sera of the channel catfish (Ictalurus punctatus) and the gar (Lepisosteus osseus) were examined for the presence of J chain. Untreated, reduced and alkylated, S-sulfonated, and cyanogen bromide cleaved samples were analyzed before and after Sephadex G-200 gel filtration by alkaline urea disc electrophoresis and other electrophoretic methods. J chains were not detected in any samples obtained from the gar. Catfish macroglobulin contained a polypeptide analogous to human J chain: it had a fast electrophoretic mobility, was coeluted with the L chain fraction, and was released only after cleavage of disulfide bonds. The isolated catfish J chain had a mol wt of 14,800 ± 500 as determined ultracentrifugally by sedimentation equilibrium in 5M guanidine-HCL and contained 6.6% carbohydrate (1 fucose, 2 mannose, 1 galactose, 2 glucosamine, and 1 sialic acid residues per chain). A comparison of amino acid analyses of human and of catfish J chain showed the latter to have a higher content of serine, glycine, phenylalanine, and histidine and a lower content of aspartic acid and arginine. Tryptic peptide maps of catfish and human J chain revealed very few common peptides. Rabbit antisera to human J chain did not cross react with catfish J chain.