Breakthrough curves of bovine immunoglobulin G (IgG) solutions from a recombinant protein G affinity membrane cartridge were significantly affected by flow velocity and feed solution concentration. Increasing the flow velocity decreased the amount of IgG bound to the membranes prior to breakthrough and broadened the breakthrough curve. Increased feed solution concentration allowed more IgG to bind prior to breakthrough but did not affect the shape of the curve. In batch incubation experiments, sorption of IgG to the membranes was slow, requiring 27 h to reach equilibrium. Sorption did not follow single-solute Langmuir kinetics. Desorption of IgG from the membranes during elution was flow rate dependent and produced multiple IgG peaks. These observations were interpreted using slow, competitive, and heterogeneous multisolute sorption of the IgG subclasses to the affinity membranes.