The invention relates to a ruthenium complex having the formulaRu.sup.2+ L.sub.1 L.sub.2 L.sub.3 Iwherein L.sub.1, L.sub.2 and L.sub.3 are the same or different are equal to a bi- or polycyclic ligand with at least two nitrogen-containing heterocycles, whereby at least one of these ligands is substituted with at least one group conferring water-solubility, and whereby at least one of these ligands is substituted, optionally via a spacer group, with at least one reactive group, and whereby the ligands L.sub.1, L.sub.2 and L.sub.3 are attached to the ruthenium via nitrogen atoms.The invention is further related to such ruthenium complexes having coupled thereto an immunologically active material, for example, antigens, haptens or antibodies and to the use of said ruthenium complexes in fluorescence spectroscopy. Specific ligands L.sub.1, L.sub.2 and L.sub.3 which are useful in the ruthenium complexes of the invention are, e.g., 2,2'-bipyridine, 1,10-phenanthroline benzbathophenanthroline or bathophenanthroline groups. Groups which are useful for conferring water-solubility on said ligand are, e.g. sulfonic acid groups which are preferably present in the form of their salts. Useful spacer groups are e.g. an alkylene group containing 1-8 carbon atoms and which is optionally substituted with --SO.sub.2 --NH--, --S--, --O--, --COO-- or --CO--NH-- groups. Useful reactive groups to which the immunologically active material is coupled, are e.g. --COOH, --I, --NH.sub.2, --NCS or --SO.sub.2 Hal groups. The ruthenium complexes according to the present invention can be detected with great sensitivity by fluorescence spectroscopy and are thus useful in fluorescence immunoassays.