18.5S and 7S immunoglobulins were purified from sera of horned sharks immunized with Brucella abortus. Both preparations had antibody activity, which was abolished more readily by reduction with dithiothreitol than with cysteine. Chromatography on Sephadex G-100 columns, in 6 M guanidine-HCl, disclosed that partial reduction with dithiothreitol cleaved all interchain bonds, whereas cysteine mainly affected the inter-subunit disulfide linkages of the 18.5S antibody molecules. Although high reactivity of sulfhydryl groups toward reducing agents was found in these immunoglobulins, the non-covalent bonds linking their chains appeared more resistant to dissociation than those of mammalian IgM. These studies suggest that in the immunoglobulins of elasmobranchs, the interchain disulfide bridges are of greater importance for stabilization of the antibody-combining sites than they are in the immunoglobulins of more recent evolutionary stages.