Method for the detection in homogeneous medium of a post-translational modification of a proteinaceous substrate catalyzed by a cell enzyme, characterized in that the post-translational modification reaction takes place in intact living cells, in that these cells comprise a heterologous expression vector coding for a fusion protein comprising the proteinaceous substrate and a first coupling domain and in that it comprises the following stages: (i) Incubation of the cells in the presence or in the absence of a compound to be tested capable of modulating the activity of said enzyme, (ii) Addition to the reaction medium of a first fluorescent compound member of a first pair of FRET partners covalently bonded to a coupling agent capable of binding specifically to the first coupling domain present on the proteinaceous substrate, (iii) Addition to the reaction medium of a second fluorescent compound member of this first pair of FRET partners, and covalently bonded to a binding domain specific to the site of the proteinaceous substrate having undergone the post-translational modification and not binding to the non-modified proteinaceous substrate, (i) Measurement of the FRET signal emitted by the sample, this signal being representative of the quantity of proteinaceous substrate having undergone said post-translational modification; and cells for the implementation of said method.