A surgical stapling instrument comprising: an end effector comprising: an elongate channel configured to operably support a surgical staple cartridge therein; and an anvil movably supported on the elongate channel between an open position and closed positions in response to an application of opening and closing motions applied thereto; and an anvil lock member cooperating with the anvil to retain the anvil in an open position when a staple cartridge has not been installed in the elongate channel and prevent the anvil from moving to a closed position until the staple cartridge has been seated within the elongate channel, wherein the anvil comprises an anvil mounting portion pivotally supported on the elongate channel, and characterized in that: the anvil lock member is biased into contact with the anvil mounting portion; the anvil mounting portion further comprises a pair of opposed anvil trunnions movably received in corresponding trunnion slots in the elongate channel; each anvil trunnion slot comprises: a locking notch for receiving the corresponding anvil trunnion therein when the surgical staple cartridge has not been seated within the elongate channel; and an arcuate slot segment for receiving the corresponding anvil trunnion therein when the cartridge has been seated within the elongate channel such that the corresponding anvil trunnion may movably traverse within the arcuate slot segment upon application of a closing motion to the anvil; and the anvil lock member biases the anvil mounting member to seat and retain the anvil trunnions in the corresponding locking notch when the surgical staple cartridge has not been seated within the elongate channel. an end effector comprising: an elongate channel configured to operably support a surgical staple cartridge therein; and an anvil movably supported on the elongate channel between an open position and closed positions in response to an application of opening and closing motions applied thereto; and an elongate channel configured to operably support a surgical staple cartridge therein; and an anvil movably supported on the elongate channel between an open position and closed positions in response to an application of opening and closing motions applied thereto; and an anvil lock member cooperating with the anvil to retain the anvil in an open position when a staple cartridge has not been installed in the elongate channel and prevent the anvil from moving to a closed position until the staple cartridge has been seated within the elongate channel, wherein the anvil comprises an anvil mounting portion pivotally supported on the elongate channel, and characterized in that: the anvil lock member is biased into contact with the anvil mounting portion; the anvil mounting portion further comprises a pair of opposed anvil trunnions movably received in corresponding trunnion slots in the elongate channel; each anvil trunnion slot comprises: a locking notch for receiving the corresponding anvil trunnion therein when the surgical staple cartridge has not been seated within the elongate channel; and an arcuate slot segment for receiving the corresponding anvil trunnion therein when the cartridge has been seated within the elongate channel such that the corresponding anvil trunnion may movably traverse within the arcuate slot segment upon application of a closing motion to the anvil; and the anvil lock member biases the anvil mounting member to seat and retain the anvil trunnions in the corresponding locking notch when the surgical staple cartridge has not been seated within the elongate channel. the anvil lock member is biased into contact with the anvil mounting portion; the anvil mounting portion further comprises a pair of opposed anvil trunnions movably received in corresponding trunnion slots in the elongate channel; each anvil trunnion slot comprises: a locking notch for receiving the corresponding anvil trunnion therein when the surgical staple cartridge has not been seated within the elongate channel; and an arcuate slot segment for receiving the corresponding anvil trunnion therein when the cartridge has been seated within the elongate channel such that the corresponding anvil trunnion may movably traverse within the arcuate slot segment upon application of a closing motion to the anvil; and a locking notch for receiving the corresponding anvil trunnion therein when the surgical staple cartridge has not been seated within the elongate channel; and an arcuate slot segment for receiving the corresponding anvil trunnion therein when the cartridge has been seated within the elongate channel such that the corresponding anvil trunnion may movably traverse within the arcuate slot segment upon application of a closing motion to the anvil; and the anvil lock member biases the anvil mounting member to seat and retain the anvil trunnions in the corresponding locking notch when the surgical staple cartridge has not been seated within the elongate channel.