[en] The behaviour of the FEU-84 photomultiplier has been studied in magnetic cross-fields up to 50 Oe, and the design of the screen which guarantees the preservation of the above device performance in conditions of a real experiment has been determined. It is elucidated that a combined screen with a steel cylinder wall 1 mm thick and with the thickness of permalloy of 300 μm, will provide reliable screening of the FEU-84 against stray magnetic fields. The necessary value of the screen lowering down beyond the FEU photocathode and, respectively, the height of cylindrical light guide installed between a glass prism and the FEU is equal to 20 mm