Our CELT-249 ligand is a highly potent fluorescent antagonist developed for hM₁R and hM₂R testing with a K_i of 133.0 nM and 11.5 nM, respectively in radioligand binding assay. The product comes in a 10 µg vial that enables the preparation of 82 mL of 100 nM working solution to test M₁R/M₂R. Our fluorescent antagonist is ideal for visualization in confocal microscopy and high-content system experiments. Our CELT-249 ligand is potentially suitable for other fluorescence-based assays.