1. A method for preparing the small-intestinal brush-border membrane of neonatal rats is described in which enzymic methods are used to remove associated polysaccharide and cell nuclei. 2. 125I-labelled IgG (immunoglobulin G) and 125I-labelled IgG Fc fragment have high specific binding and low non-specific binding to brush borders prepared in this way. F(ab)'2 fragment however, does not bind, indicating the existence of a specific receptor for the Fc fragment of IgG. The receptor system is saturable, and the affinity (KA) for the binding of rat IgG was determined by both equilibrium and kinetic methods. 3. The binding of heterologous IgG species (human and bovine) was compared and demonstrated a close similarity between human IgG and rat IgG in their receptor affinities. 4. Kinetic results are presented that are consistent with previously proposed models of ligand-induced receptor aggregation.