a disposable loading unit is provided which includes a tool assembly and a body portion . a drive assembly is movably positioned at least partly within the body portion and is operably associated with the tool assembly to operate the tool assembly . a locking mechanism is provided for maintaining the drive assembly in a prefired position prior to and during attachment of a dlu to a surgical instrument . the locking mechanism includes a locking member and a locking member actuator . the locking member actuator is movable to move the locking member to an unlocked position in response to linear insertion of an insertion tip of the dlu into a surgical instrument .