Human J chain isolated from the plasma of a patient with Waldenstrom's macroglobulinemia was subjected to extended and limited digestion with trypsin and chymotrypsin. The digests were fractionated by combination of column chromatography and high voltage paper electrophoresis. Peptide purity was established by their amino acid analysis and a single amino terminal residue. All the necessary peptides which would provide the total primary structure of molecule were thus obtained. © 1977, American Chemical Society. All rights reserved.