Receptor-ligand interactions have been extensively used to establish the specificity and binding profiles for chemokine receptors and their ligands. Furthermore, receptor-ligand binding assays are used extensively for drug screening. The Scintillation Proximity Assay (SPA) is now a well established screening technology and a considerable number of SPA based receptor screening assays have been run by pharmaceutical companies. This homogeneous radioisotopic technique offers considerable advantages in terms of convenience, safety, and cost. The method utilizes scintillant-containing microspheres that are chemically treated to enable the coupling of molecules (e.g., antibodies, receptors, and enzymes) to their surface. Assays are carried out in aqueous buffers using radioisotopes that emit low energy radiation which is easily dissipated in an aqueous environment.