Human J chain is a polypeptide folded within the polymeric structure of the immunoglobulin. J chain isolated from human polymeric IgA and IgM are identical by criteria of composition, peptide maps and antigenicity. Human J chain has been established as distinct from all other component chains of polymeric IgA and IgM. It has a unique primary structure. Antisera to light chains or heavy chains of human Immunoglobulins do not react with J chain. J chain is released from dimeric human myeloma IgA by sulphitolysis and purified by ion exchange chromatography in urea and gel filtration in guanidine. Its purity is tested in alkaline urea polyacrylamide gel electrophoresis, in immunodiffusion and immunoelectrophoresis, showing it to be free of IgA, heavy or light chain contaminants.