The L polypeptide chains of certain Bence-Jones proteins of group I have been found in three forms: monomers of molecular weight of about 20,000, dimers which monomerize in dissociating solvents, and dimers which are stable in such solvents. The L polypeptide chains of some Bence-Jones proteins of group II were found to occur naturally only as stable dimers. The L chains of normal human gamma-globulin have been obtained in a reduced unalkylated form, and a fraction of these chains was found to form stable dimers under oxidizing conditions. It is suggested that a single disulfide bond is involved in stabilization of the dimer. In experiments on the reconstitution of 7S gamma-globulin, it was found that stable dimers of L polypeptide chains did not associate appreciably with H(gamma) chains to form a soluble product. L chains in the monomeric form, both of a reduced alkylated Bence-Jones protein and of reduced unalkylated gamma-globulin, combined with H(gamma) chains to form a 7S product. After hydrolysis with papain, the 7S material containing the Bence-Jones L chains yielded fragments comparable to the fragments of papain-treated myeloma proteins. As indicated by spectrofluorometric measurements, dissociable dimers and stable dimers of the L chains of a Bence-Jones protein both underwent identical thermally induced transitions in the temperature range 48-58 degrees C. When L polypeptide chains were present in reduced alkylated gamma-globulin or reduced alkylated S fragments, no transition occurred until 65 degrees C, the coagulation temperature of gamma-globulin and S fragments. Above this temperature, L chains were released into solution. These experiments suggested that free L chains and L chains bound to H(gamma) chains have different conformational stabilities.