Uses of ethyl 5,11-dihydroindolo[3,2-b]carbazole-6-carboxylate as a fluorescent ligand probe, preferably in a binding assay for quantitative analysis in combination with a recombinant aryl hydrocarbon receptor (AhR) protein. A method for detection or quantitative analysis of suspected aryl hydrocarbon receptor (AhR) ligands in a sample, the method comprising the steps of: (a) providing a sample possibly containing at least one known or unknown AhR ligand; (b) mixing said sample with a composition comprising a recombinant AhR protein bound to ethyl 5,11-dihydroindolo[3,2-b]carbazole-6-carboxylate; and (c) determining the presence or the total amount of said at least one known or unknown AhR ligand in the sample by fluorescence spectroscopy. Use e.g. for screening of unknown compounds to determine their binding ability to AhR; for analyzing unknown chemical mixtures to assess the presence of ligands able to bind to AhR; for screening of Polycyclic Aromatic Hydrocarbon (PAHs) from water, food or sediment; for sc
