A method of authentication of a terminal generating a magnetic field, by a transponder including an oscillating circuit from which a D.C. voltage is generated, wherein the transponder: receives first data relative to the current in an oscillating circuit of the terminal, measured by the terminal for a first value of the resistive load of the transponder; and exploits these first data and second data relative to the level of said D.C. voltage, respectively measured for the first resistive load value and for a second resistive load value.