Lanthanide luminescence and in particular sensitised emission from both europium (III) and terbium (III) is reviewed. The use of both europium and terbium as biological markers is covered, concentrating on time-resolved fluoroimmunoassays (TR-FIA), and more recently nucleic acid hybridisation assays. A new approach is described for the design of cooperative ligands that enhance europium luminescence in aqueous solution. This involves a three-component system comprised of the europium. ion, a sensitising ligand, 1,10-Phenanthroline-2,9- biscarboxylic acid, and a separate non-sensitising ligand, N-(butylacetamido)- ethyl enediamine-N, N, N'-tri acetate. The synthesis of a novel class of europium sensitiser is described 5-(Hexanoylphenanthridinium)-2,9-biscarboxylic acid- 1,1 O-phenanthroline. A homogeneous method for identifying the presence of a single strand of DNA is described which employs 5-(Hexanoylphenanthridinium)-2,9-biscarboxylic acid- ,10- phenanthroline as a cooperative sensitiser of europium (M) luminescence.