The digestion of a human IgG1 K myeloma protein with pepsin in the presence of 8M-urea was observed to produce a fragment, designated Fb'2, which differed from the products of aqueous peptic digestion and from other characteristic immunoglobulin digestion products. 2. Fragment Fb's was also found when two other IgG1/K proteins were treated similarly. 3. Sedimentation-equilibrium studies showed the mol.wt. of fragment Fb'2 to be 56800. 4. On reduction, two equivalents of each of three peptides were released from fragment Fb's; these were characterized by N- and C-terminal determinations and by amino acid sequencing. 5. Fragment Fb'2 was shown to consist of the constant regions of both light chains, from residue Ile-117 to the C-terminus, and the CH1 domains and hinge region of the heavy chains, from residue Val-113 to residue Met-252, with a gap of five residues within the intrachain disulphide loop, between residues Leu-174 and Tyr-180.