The Rh saline and albumin type antibodies of human serum show distinct chemical and immunologic specificities. These two antibodies do not appear to be dissociation or aggregation products of each other. Mercaptan dissociation of the saline agglutinin and papain digestion of the albumin type antibody leads to their being converted into molecules which are unable to effect their usual agglutinin reactions but which are still capable of combining with antigen.