The 17S and 7S immunoglobulins of the smooth dogfish, Mustelus canis, have been characterized with respect to molecular weights, chain structure, amino acid composition, and carbohydrate content. The 17S protein had a molecular weight of 982,000; that of the 7S protein was 198,000. Both proteins had similar amino acid compositions and a carbohydrate content of approximately 8 percent. Light and heavy chains from both immunoglobulins had molecular weights of 20,000 and 72,000 respectively. Apparently both immunoglobulins belong to the same class; this class resembles the gammaM-immunoglobulins of higher vertebrates.