Sorption of the four subclasses of human immunoglobulin G (hIgG) to recombinant protein G immobilized to microporous membranes was examined to further the understanding and characterization of this medically-important system. Using batch incubation, sorption of the individual hIgG subclasses was measured in competitive and noncompetitive experiments. Individually, all subclasses had very similar sorption rates and equilibrium capacities. In contrast, for mixtures of the subclasses, binding was distinctly different, with strong competitive binding occurring: as the system approached equilibrium, significantly more hIgG1 and hIgG3 bound to the membrane than did hIgG2 and hIgG4. A kinetic and equilibrium model was able to successfully simulate the binding of hIgG1, hIgG2, and hIgG4 but not hIgG3. The results are relevant to the healthcare and biotechnology industries: (1) the diagnosis and treatment of autoimmune diseases, (2) mitigation of rejection in recipients of organ transplants, and (3) production of monoclonal antibodies for use as therapeutic biopharmaceuticals.