Knowledge of a receptor’ pharmacology provides information which is often essential to unraveling the biological function of that receptor. The chemokine system represents a striking example as most receptors in the family bind multiple chemokines, often with complex overlapping selectivities. Two basic methodologies are used to characterize the binding properties of a receptor. In saturation protocols, the binding of a given ligand, usually labeled to allow detection, is determined over a range of concentrations appropriate for that ligand. Competition experiments measure the ability of varying concentrations of an unlabeled ligand to inhibit the binding of a fixed concentration of a labeled ligand. Although saturation protocols have the advantage of theoretical simplicity, they have three significant disadvantages: