The study investigates the behaviour of three-layered cantilever sandwich beams filled with magnetorheological fluids (MRFs) differing in the iron particle content by volume. Outer layers are made of aluminium, the space between them is sealed with silicone rubber. Two types of beams are considered: fully filled beams and partially filled beams, subjected to the magnetic field. The aim of the study is to determine stiffness and damping characteristics in relation to the magnetic field strength and the actual location where the magnetic field acts upon the beam. For this purpose, measurements have been taken of the beam free vibration response for various magnetic field strength levels and for various positions of the electromagnet located along the beam axis. Basing on the developed measurement data processing algorithm, the influence of the vibration amplitude on the natural frequency and a dimensionless damping coefficient have been determined. Finally, the equivalent natural frequency and the dimensionless damping coefficient have been derived accordingly, and the stiffness and damping ratio have been determined in function of the magnetic field strength, the electromagnet position and the MRF iron particle content by volume.