Antibodies have played a key role in receptor research. These immunoglobulms (Ig) frequently have greater affinities and selectivities for receptors than pharmacological probes. They are often the only probes available that can discern proteins with similar structural and pharmacological properties, such as distinct isoforms or splice-variants of a receptor family. Furthermore, antibodies may be utilized to determine a wide range of a receptor’ characterrstrcs: its apparent molecular mass by immunoblotting; its interaction with other proteins, covalent modifications, and turnover rates by immunoprecipitation; its localization in tissues, cells, and subcellular domains by immunohistochemistry, immunocytochemtstry, and immunoelectron microscopy; its primary structure by means of screening complementary DNA (cDNA) expression libraries; and its functional properties by virtue of some antibodies acting as receptor agonists/antagonists.