Prototypes of potentiometric immunosensors for human immunoglobulin G (hIgG) determination are presented. Such biosensors were obtained by covalent binding of monomolecular hIgG layer directly to the surface of ion-selective membranes sensitive to hydrogen and ammonium ions. For the hIgG determination two-step competitive procedure with the use of anti-hIgG-urease conjugate was applied. The biocatalytic action of immunologically bound urease was the source of the analytical signal of the immunosensors. Total time of analysis including incubation, measurement and regeneration steps was around 1/2 h. The highly stable and sensitive biosensors enable multiple determination of hIgG in the clinically important range of concentration.