A surgical stapling instrument is disclosed, having upper and lower elongate jaws, one of which is adapted to receive a staple magazine, the other jaw being adapted to receive an anvil or to serve as one itself. A pusher bar and knife assembly including a pair of pusher bars and a central knife carrier moves along the jaws to eject staples from the magazine sequentially and to form laterally spaced staple rows in tissue gripped between the jaws while the knife cuts the tissue along a line between the staple rows. The instrument includes structure for locally supporting the jaws in the region of the forward ends of the pusher bars as these elements move along the jaws to resist forces which arise during staple ejection and shaping and which tend to vertically separate or laterally distort the jaws, or both. The magazine and the pusher bar and knife assembly may together constitute a unitary disposable cartridge. One jaw may have tabs or lugs for gripping the other jaw to provide additional lateral stability. A detent may also be provided to prevent operation of the instrument while the jaws are open.