We present a novel haptic interface for musical keyboards aimed to reproduce the dynamic behavior of grand-piano action mechanism based on magneto-rheological fluids. The intrinsic dissipative characteristic of MR fluids ensures passivity making them appealing for the use in haptic interface. The interface is composed of three main parts: an active damping system, a mechanical structure, sensors and electronics needed for control. The damping system uses a thin magnetic plate to shear the MR fluid contained into the gap. When a magnetic field is applied, the fluid apparent viscosity increases generating a resistive force against the motion of the plate set by the user. An experimental device used for the MR fluid behavior identification is also presented.