The elasmobranch Mustelus canis has been shown to produce antibodies to Limulus hemocyanin. The serum of both normal and immunized M. canis contains immunoglobulins having sedimentation coefficients of approximately 7S and 17S. Antibody activity was found in the 17S immunoglobulin which may be dissociated to 7S components with concomitant loss of activity. Both 17S and 7S serum, immunoglobulins were antigenically identical. They consisted of light and heavy chains present in amounts comparable to those of higher vertebrates. Peptide maps indicated that the light chains had an entirely different primary structure than the heavy chains, but that the corresponding chains of 7S and 17S dogfish serum immunoglobulins were similar in primary structure. The heavy chains appeared to resemble the n chains of immunoglobulins of higher vertebrates in their starch gel electrophoretic behavior. It is suggested that the elasmobranch M. canis may have only one major class of immunoglobulins resembling that of macroglobulins (gammaM-immunoglobulins) seen in higher vertebrates. The results indicate that the multichain structure of antibodies is an ancient evolutionary development.