Our red-emitting fluorescent GTPγS demonstrates outstanding performance in determining the functional activity of GPCR modulators in both agonist and antagonist modes. The assay requires only low concentrations (in the nM range) of CELT-503, enabling up to 20 plates of 96 wells to be processed using a single vial. The emission in the red spectrum minimizes autofluorescence effects and makes it highly suitable for multiplexing assay developments. CELT-503 addresses the limitations associated with the use of conventional [³⁵S]GTPγS, significantly broadening the range of assays in which it can be applied.