A self contained gas powered endoscopic surgical apparatus is provided for placing lateral lines of surgical fasteners into body tissue. The apparatus includes an anvil member and a surgical fastener cartridge member mounted to the distal end of an elongated endoscopic portion. A tubular collar of the endoscopic portion moves distally to engage the anvil member and bias the anvil member and the cartridge member into cooperative alignment, thereby clamping body tissue to be fastened between the anvil member and the cartridge member. A self contained pneumatic system is disposed in the surgical apparatus and is actuable to eject and/or form the surgical fasteners in the clamped body tissue. The apparatus further comprises a locking mechanism for preventing approximation of the anvil member and the cartridge. In one embodiment a clamping interlock is shown which prevents approximation of the jaws when the jaws are either misaligned or improperly inserted into the instrument. In a second embodiment a clamping interlock is shown which directly engages the tubular collar preventing approximation of the jaws when a cartridge member is not fully inserted. In a third embodiment a clamping interlock is shown which directly engages the tubular collar when an unfired cartridge is not present or properly inserted into the instrument.