Abstract: The present invention pertains to building blocks, especially for erecting shielding walls in nuclear facilities. Each building block is made of a cubeshaped metal jacket consisting of steel plate, especially special steel, which jacket is closed on all sides and is filled with concrete. To facilitate the handling and to permit the erection of shielding walls with building blocks in fixed positions, the top surfaces and the bottom surfaces (2, 4) of the jacket (1) are provided with bulging formations in the form of projections (3) and depressions (4) [sic, (5)] which engage with each other between adjacent building blocks to mutually fix the positions of the building blocks.