A tool assembly comprising: a cartridge assembly including a staple cartridge and an actuation sled positioned within the staple cartridge, the actuation sled movable from a retracted position to an advanced position to eject staples from the staple cartridge; and an anvil assembly pivotally secured to the cartridge assembly and movable in relation to the cartridge assembly between open and clamped positions, the anvil assembly including an anvil body, an anvil cover, and a lockout member, the anvil body including an anvil member having an inner surface defining a plurality of staple deforming pockets, an outer surface defining a longitudinal groove, a central knife slot extending between the inner and outer surfaces, and a through bore extending between the inner and outer surfaces, the anvil cover defining a cutout and being secured to the anvil body to define a longitudinal channel within the anvil assembly, the lockout member having a horizontal portion and a vertical portion, wherein the lockout member is movable from a first position in which the horizontal portion of the lockout member is positioned within the longitudinal channel of the anvil assembly to a second position in which the horizontal portion of the lockout member is positioned within the cutout of the anvil cover of the anvil assembly, wherein the tool assembly further includes a biasing member, characterised in that the biasing member urges the lockout member towards the first position; wherein the vertical portion of the lockout member is positioned to engage the actuation sled when the actuation sled is in its retracted position; and wherein engagement between the vertical portion of the lockout member and the actuation sled moves the lockout member from the first position to the second position as the anvil and the cartridge assemblies are moved to the clamped position. a cartridge assembly including a staple cartridge and an actuation sled positioned within the staple cartridge, the actuation sled movable from a retracted position to an advanced position to eject staples from the staple cartridge; and an anvil assembly pivotally secured to the cartridge assembly and movable in relation to the cartridge assembly between open and clamped positions, the anvil assembly including an anvil body, an anvil cover, and a lockout member, the anvil body including an anvil member having an inner surface defining a plurality of staple deforming pockets, an outer surface defining a longitudinal groove, a central knife slot extending between the inner and outer surfaces, and a through bore extending between the inner and outer surfaces, the anvil cover defining a cutout and being secured to the anvil body to define a longitudinal channel within the anvil assembly, the lockout member having a horizontal portion and a vertical portion, wherein the lockout member is movable from a first position in which the horizontal portion of the lockout member is positioned within the longitudinal channel of the anvil assembly to a second position in which the horizontal portion of the lockout member is positioned within the cutout of the anvil cover of the anvil assembly, wherein the tool assembly further includes a biasing member, characterised in that the biasing member urges the lockout member towards the first position; wherein the vertical portion of the lockout member is positioned to engage the actuation sled when the actuation sled is in its retracted position; and wherein engagement between the vertical portion of the lockout member and the actuation sled moves the lockout member from the first position to the second position as the anvil and the cartridge assemblies are moved to the clamped position.