The interest in the role of metabotropic glutamate receptor 4 (mGlu 4) in CNS related disorders has increased the need for methods to investigate the binding of allosteric drug candidates. Our aim is to present the first fully characterized in vitro binding assay of mGlu 4 positive allosteric modulators (PAMs). Results suggest that mGlu 4 PAMs have characteristic co-operative binding with orthosteric glutamate, which offers a notable insight to the further development of mGlu 4 targeted therapies.