With antisera for 4 crystalline Bence-Jones proteins, it is demonstrated that there are 2 immunologically distinct groups of Bence-Jones protein. Bence-Jones proteins, which had been analyzed for their % content of the more important amino acids, were studied with the aid of these antisera. Proteins of the same composition from the standpoint of amino-acid content may be different in their immunologic properties and proteins that are different in their % content of the various amino acids may be immunologically identical. A patient mav eliminate both types of Bence-Jones protein at the same time.