A method for detecting the internalization of a transmembrane protein of interest expressed at the surface of a cell, comprising the following steps: 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 a modulating agent selected from: a. a fluorescent or nonfluorescent FRET acceptor compound compatible with said fluorescent metal complex, the final concentration of which in the reaction medium is greater than 10−7M; b. a reducing agent, the redox potential of which is less than +0.1 V and preferably between 0.25 and 0.75 V; c. an agent which binds specifically, by noncovalent bonding, with the fluorescent metal complex; d. a metal ion which competes with the rare earth so as to form a nonfluorescent 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 said compound is a fluorescent acceptor compound; e) comparing the signal measured in step d) with a reference signal measured on cells having been subjected only to steps a) and c).