Ligand-label conjugates which are an oligopeptide of 5 to 100 amino acid residues, bonded to a ligand or receptor, which contain a plurality of chemiluminescent or fluorescent labels and a plurality of polyoxoanions of sulfur or phosphorus are useful for immunoassays. Such conjugates are hydrophilic and exhibit very low nonspecific binding, thereby significantly increasing the signal to background ratio in immunoassays.