A Eu(III)-macrocycle-mono-isothiocyanate, Quantum Dye®, has been coupled to a monoclonal antibody against 5BrdU. Since Quantum Dyes do not undergo concentration quenching, the coupling conditions were optimized to achieve the maximum number of Eu(III) macrocycles bound to the antiBrdU, without decrease in solubility or loss of antigen-binding ability. In order to optimize the coupling conditions, a colorimetric method for the quantitation of the Eu(III)- macrocycle-mono-isothio-cyanate has been developed. A simple mixture composed of an ethanolic solution and a Gd(III)- containing aqueous solution is now used to provide lanthanide enhanced luminescence, LEL. Under LEL conditions, the specific binding of Eu(III) macrocycles to apoptotic cells has been observed in both aqueous and mounted slide preparations. A comparison between measurements of the same LEL model system, obtained in both time-gated luminescence and standard fluorescence modes, has demonstrated that time- gating significantly improves the signal to noise ratio.