Our potent C5a receptor fluorescent antagonist shows high affinity for the C5aR (EC₅₀ = 24.89 nM by saturation binding in Chem1 transfected cells), good potency (K_B = 5.8 μM by calcium flux assay) and good competition with the endogenous ligand C5a. It has been validated in flow cytometry competition binding assays using Chem1 transfected cells. It is potentially suitable for other fluorescence-based assays.