Orientation on phase borders and stability to surface denaturation of samples of rabbit and human immunoglobulin G (IgG) were studied by monomolecular layer method. Surface denaturation of protein was judged by the position of curves pressure--area (pi-A) with an increase of NaCl concentration in solution from zero to 2M. Native IgG molecules have a planar orientation on the phase border and the zone of stability to surface denaturation in the concentration range of NaCl lies within 0-0.5 M. Such stability zone was not observed after the reduction of interchain disulfide bond in the hinge region of the molecule and for proteolytic fragments of IgG (F(ab')2, Fab' and Fc). Both proteolytic fragments and Fab and Fc regions in te molecules of reduced IgG are oriented vertically to the phase border. It has been found that the decisive role in the planar position of the IgG molecules on the interface and in their stability to surface denaturation is played by the disulfide bond in the hinge region, which stabilizes the conformation of IgG molecules.