A surgical end effector, comprising: a surgical staple cartridge; a first jaw configured to operably support the surgical staple cartridge, the surgical staple cartridge operably supporting a cam assembly therein, the cam assembly being axially movable through the surgical staple cartridge to eject surgical staples therefrom when the cam assembly is moved from an unfired position to a fired position; a firing member supported for axial movement within the surgical staple cartridge between a starting position proximal to the surgical staple cartridge and an ending position within the staple cartridge upon applications of firing and retraction motions thereto, the firing member body comprising two lateral sides; and a lockout system supported within said surgical end effector, characterised by said lockout system comprising a U-shaped spring, said U-shaped spring comprising first and second locking portions and a central spring portion, wherein each locking portion extends from the central spring portion, said locking portions biased laterally into lateral retaining engagement with said firing member, wherein the first locking portion is configured to retainingly engage one side of the firing member body and the second locking portion is configured to retainingly engage the other side of the firing member body, when said firing member is in said starting position to thereby retain said firing member in said starting position, and wherein said locking portions are configured to be laterally biased out of said lateral retaining engagement when contacted by the cam assembly of the surgical staple cartridge that is seated within said first jaw and the cam assembly is in the unfired position. a surgical staple cartridge; a first jaw configured to operably support the surgical staple cartridge, the surgical staple cartridge operably supporting a cam assembly therein, the cam assembly being axially movable through the surgical staple cartridge to eject surgical staples therefrom when the cam assembly is moved from an unfired position to a fired position; a firing member supported for axial movement within the surgical staple cartridge between a starting position proximal to the surgical staple cartridge and an ending position within the staple cartridge upon applications of firing and retraction motions thereto, the firing member body comprising two lateral sides; and a lockout system supported within said surgical end effector, characterised by said lockout system comprising a U-shaped spring, said U-shaped spring comprising first and second locking portions and a central spring portion, wherein each locking portion extends from the central spring portion, said locking portions biased laterally into lateral retaining engagement with said firing member, wherein the first locking portion is configured to retainingly engage one side of the firing member body and the second locking portion is configured to retainingly engage the other side of the firing member body, when said firing member is in said starting position to thereby retain said firing member in said starting position, and wherein said locking portions are configured to be laterally biased out of said lateral retaining engagement when contacted by the cam assembly of the surgical staple cartridge that is seated within said first jaw and the cam assembly is in the unfired position.