A surgical fastener applying apparatus for applying fasteners to body tissue including a cartridge receiving half-section defining an elongated channel member configured to releasably receive a stationary housing of a firing assembly. The stationary housing is dimensioned to releasably receive a single use loading unit therein, and includes a lockout structure that prevents insertion of the stationary housing into the cartridge receiving half-section if the single use loading unit is not mounted in the stationary housing.