Fluorescence immunoassay of biological fluids (for example, blood samples) is discussed. We attempt to chart present methods of assay as well as new possibilities. Different fluorescent probes, their detection limit, and methods for reduction of background are discussed; methods for separating the free and bound fraction are also reviewed. Special consideration is given to the possibilities of enhancing sensitivity by developing both instruments and chemical methods, and in particular to the possibilities inherent in time-resolved fluorometric applications and to the use of metal chelates in this application.