The radioligand binding assay is a useful tool to characterize peptide receptors and to study the interaction of endogenous and synthetic peptides with those receptors. The most commonly used method is the membrane filtration technique that involves binding of the radioligand to membranes that contain the receptor of interest. The bound and free radioligand are then separated by rapid filtration through glass fiber filters. This technique can be used for many different peptides as long as a high-affinity radioligand is available.