Antigen-binding activity of five new monoclonal antibodies (McAb) to class-specific determinants of human IgG was investigated. As shown by competitive-enzyme immunoassay these McAb recognize five different epitopes on IgG molecule. Three epitopes (5A9, 4D6 and 3D3) were localized on Fc-fragment, while two others (5H3 and 5F2) could be revealed neither on Fab- nor on Fab2-fragments. Four McAb (5H3, 5F2, 5A9, 3D3) recognize the epitopes that are expressed similarly on IgG in the liquid phase or adsorbed in the solid phase. The expression of the fifth epitope (4D6) is increased substantially after immobilization on plastic surface. The antigen-binding activity of an McAb remains unchanged after covalent coupling to horseradish peroxidase (HRPO). Three McAb only (5H3, 5F2, 5A9) retain their reactivity profile after solid phase immobilization on plastic surface. Combined usage of immobilized 5H3 McAb and HRPO-coupled 3D3 McAb permits designing of immunometric assay for total IgG determination irrespective of subisotype composition. The applicability of HRPO-coupled 3D3 and 5A9 McAb for IgG-autoantibodies and antiviral IgG antibodies detection is demonstrated. The expression and conformational variability of class-specific determinants of IgG is discussed.