The instant invention provides for methods for detecting the internalization of a transmembrane protein of interest expressed at the surface of a cell. More specifically, the methods involve (a) labelling the protein of interest with a fluorescent metal complex, the lifetime of which is greater than 0.1 ms, (b) adding to the reaction medium a composition capable of causing the internalization of the protein of interest, (c) adding to the reaction medium (1) a modulating agent which is a fluorescent FRET acceptor compound compatible with the fluorescent metal complex, the final concentration of which in the reaction medium is greater than 10−7M; or (2) a reducing agent, the redox potential of which is less than +0.1 V and preferably between 0.25 and 0.75 V; or (3) an agent which binds specifically, by non-covalent bonding, with the fluorescent metal complex; (d) adding a metal ion which competes with the rare earth so as to form a non-fluorescent metal complex; (d) measuring the luminescence emitted by the reaction medium at the emission wavelength of the fluorescent metal complex and/or at the emission wavelength of the modulating compound when the compound is a fluorescent acceptor compound; and (e) comparing the signal measured in step d) with a reference signal measured on cells having been subjected only to steps a) and c).