TR-FRET (Time-Resolved Fluorescence Resonance Energy Transfer) combines advantages of time resolved fluorescence and FRET. Energy is transferred from a long-lived lanthanide chelate, such as Europium to an acceptor molecule with short-lived fluorescence. Eu-labeled donors have a longer lasting fluorescence signal than standard fluorescence dyes, which allows for measurements to be taken after decay of sample background autofluorescence, leaving only the TR-FRET signal, essentially free of background interference. Streptavidin Europium conjugate has been optimized in Eu/protein labeling ratio to ensure high signal and uncompromised streptavidin function. This product is stable in buffers with pH from 5.5 to 9.0, containing up to 20mM EDTA or DTT, and up to 1% Triton X-100 without significant loss of the luminescent activity of the conjugate.