The pFh fragments from the hinge region of human IgG3 Kuc and Sur are able to fold into a compact structure, thereby giving proteins with secondary (supersecondary) structure, mainly represented by the left-handed polyproline II helix. It has been shown that thermal denaturation of a compact pFh from the hinge region of IgG3 Kuc and Sur comprises two stages. At the first stage, the compact protein structure unfolds according to the all-or-none model with retention of the secondary structure. At the second stage, the lefthanded polyproline II helix, composed of four separate cooperative blocks formed of strands with a high content of proline residues, melts itself. A polyproline conformation of the secondary structure and alarge number of disulfide bonds between the chains determine a high specific enthalpy of denaturation and a high thermal stability, respectively.