A surgical fastener applying apparatus comprising: an anvil half-section; a cartridge receiving half-section defining a channel member; a single use loading unit configured to be received in the channel member; and a firing assembly configured to be received in the channel member, the firing assembly including a cam bar and a knife actuating bar movably positioned within the channel member, the cam bar being configured to move through the loading unit upon actuation of the firing assembly, the firing assembly further comprising a lockout structure configured to prevent insertion of the single use loading unit into the channel member into operative engagement with the firing assembly after the firing assembly is mounted in the channel member independently of the single use loading unit; characterised in that the lockout structure is movable along the channel member between a first position which prevents operative engagement between the single use loading unit and the firing assembly and a second position which allows operative engagement between the single use loading unit and the firing assembly, wherein the channel member comprises an indentation for receiving at least part of the lockout structure to retain the lockout structure in the first position when the firing assembly is inserted into the channel member, to prevent movement of the lockout structure to the second position, wherein when the lockout structure is in the first position, the cam bar is in a forward position relative to the firing assembly preventing entry of a proximal end of the single use loading unit into a distal end of the firing assembly. an anvil half-section; a cartridge receiving half-section defining a channel member; a single use loading unit configured to be received in the channel member; and a firing assembly configured to be received in the channel member, the firing assembly including a cam bar and a knife actuating bar movably positioned within the channel member, the cam bar being configured to move through the loading unit upon actuation of the firing assembly, the firing assembly further comprising a lockout structure configured to prevent insertion of the single use loading unit into the channel member into operative engagement with the firing assembly after the firing assembly is mounted in the channel member independently of the single use loading unit; characterised in that the lockout structure is movable along the channel member between a first position which prevents operative engagement between the single use loading unit and the firing assembly and a second position which allows operative engagement between the single use loading unit and the firing assembly, wherein the channel member comprises an indentation for receiving at least part of the lockout structure to retain the lockout structure in the first position when the firing assembly is inserted into the channel member, to prevent movement of the lockout structure to the second position, wherein when the lockout structure is in the first position, the cam bar is in a forward position relative to the firing assembly preventing entry of a proximal end of the single use loading unit into a distal end of the firing assembly.