It is intended to provide novel labeling reagents characterized by having a group capable of binding to a substance to be labeled (for example, a biological substance, a physiologically active substance, etc.), easily forming a complex together with a
rare earth ion, the complex being stable in an
aqueous solution, and having a sufficient
fluorescence intensity and a long
fluorescence life time regardless of buffer types; complexes composed of the above labeling
reagent with a
rare earth ion;
fluorescence labels containing the above complex; a
fluorescence assay method using the above fluorescent
label; etc. Namely, labeling reagents comprising a compound having a 2,2′:6′,2″-tripyridine skeleton or a 2,6-dipyrazolopyridine skeleton and having a group capable of binding to a substance to be labeled (for example, a biological substance, a physiologically active substance, etc.) and a group capable of forming a complex together with a
rare earth ion; complexes composed of the above labeling
reagent with a rare earth ion; fluorescence labels containing the above complex; a fluorescence labeling method using the above complex as a
label; and a
fluorescence assay method using the above fluorescent
label.