The Radio Science Experiments of the BepiColombo mission will enable substantial improvement of the knowledge of Mercury's orbit and rotation, and the relativistic dynamics in the solar system. A fundamental support to the spacecraft tracking data will be given by the Italian Spring Accelerometer (ISA). This is a three-axis accelerometer devoted to the measurement of the non-gravitational perturbations acting on the Mercury Planetary Orbiter (MPO), whose knowledge is important in order to fully exploit the quality of the tracking data. The intrinsic noise level of the instrument that will be onboard MPO, 10 - 9 m / s 2 / Hz in the 3 × 10 - 5 to 10 - 1 Hz frequency range, guarantees the fulfilment of the RSE requirements. The main scientific and technological features of the instrument are discussed, together with its current error budget, experimental activities and foreseen calibration strategies.