Abstract: A building construction modular system utilizing interlocking building blocks made of aggregate material and having opposite side faces vertically and horizontally offset from the main body portion of the block. A structure, such as a wall, is erected by laying superimposed horizontal courses of vertically interlocked blocks, the blocks of a course being interlockingly supported by the blocks of the subjacent course. Each block element is provided with a vertically disposed passage or aperture and with a vertically directed recess at each lateral end of the block where it interlocks with an adjoining block element and the integrally formed transverse walls joining the side faces of each block are appropriately apertured such that, after the wall has been erected, cement is poured into the block structure from the tops of the blocks on the upper course, so as to flow vertically and horizontally through the passages and apertures within the blocks.