A fragment termed fragment Fc′ and a related fragment termed fragment pFc′ produced by the actions of papain and pepsin respectively on human immunoglobulin G have been isolated and characterized. Amino acid analyses and experiments utilizing cyanogen bromide to cleave the methionyl bonds of the Fc′ and pFc′ fragments make it possible to locate both fragments within the known chain structure of the immunoglobulin G molecule. The pFc′ fragment is probably a non-covalently linked dimer situated at the C-terminal end of the molecule, containing about 232 amino acid residues and having a molecular weight of 26000. The Fc′ fragment is a similar dimer of about 182 residues extending from near residue 14 to near residue 105 (numbered from the C-terminal end) of the γ-chain and has a molecular weight of 21000.