A FRAGMENT in normal human urine related specifically to the heavy chains of immunoglobulin G (IgG) was found1 by gel diffusion and immunoelectrophoretic analysis to be antigenically similar to the Fc′ fragment produced by papain digestion of the IgG molecule. The presence of a fragment of this kind in urine has been confirmed2,3, but no attempt has yet been made to purify and chemically characterize it. It has been proposed4 (with experimental support5) that the Fc′ fragment obtained by papain digestion of human IgG is a dimeric subunit extending from near residue 14 to near residue 105 (numbering from the C-terminal end) of the γ-chain. We now report the isolation of urinary Fc′ fragment, together with physicochemical data confirming its close similarity to enzymatic Fc′ fragment.