G-protein coupled receptors are primary targets in high throughput compound screening against a variety of disease conditions. To help accomplish this, many analytical techniques are employed using a diverse set of both cell-based and cell-free assay systems. The homogeneous technique of fluorescence polarization is becoming an increasingly popular detection method, measuring the change in molecular tumbling or rotational diffusion of a fluorescently labeled compound usually due to changes associated with its molecular mass. To demonstrate these principles, a peptide ligand for the mu-opioid receptor, Dermorphin, has been labeled with Cyä3B, a red-shifted cyanine fluorophore, and evaluated in a fluorescence polarization assay read on the FARCyteä fluorescent plate reader (Amersham Biosciences) in both 384-well and 1536-well format. The data are compared to those obtained using the radiometric scintillation proximity assay (SPA).