An experiment on the propagation of a plane-polarized SHF-ranged electromagnetic wave in magnetic colloids containing magnetite particles was performed. It is found that the action of the magnetic field aligned with the wave on such media results in turning (rotation) of the wave polarization plane (Faraday effect). Moreover, this effect is pronounced only at exceeding a certain critical volume concentration of magnetite. Some peculiarities of the phenomenon have been found in structurally ordered magnetic colloids which could be attributed to the alteration of their structure under the magnetic field action.