Earlier, the electron microscopy and hydrodynamic studies revealed the transformation of the globule-like form of the human (h) IgG3 Kus hinge into a rod-like shape under non-denaturing perturbations [Eur. J. Biochem. 190 (1990) 393]. In this work, it is shown with the differential scanning calorimetry (DSC) that the melting of a globule-like form of the hIgG3 Kus hinge is a co-operative process. The 'two-state' model accepted for small globular proteins well describes the transition. Thus, in the hIgG3 Kus molecule, the core hinge folds into the thirteenth globular domain. The model of folding of four double poly-L-proline (PLP) helices of the core hinge into the compact tertiary structure similar to 'a folded camp bed' is suggested.