A bag for a circuit of a biological liquid treatment installation, said circuit being provided with a press comprising a first shell and a second shell and with said bag being inserted between said first shell and said second shell, characterized in that said bag comprises: - two flexible films (65, 66) joined to each other by a seal delimiting a closed outline which form a zone for treatment of said liquid according to a closed outline, connectors for liquid emerging on the inside and on the outside of at least one side (68, 69, 70) of said outline and a network for conveying liquid between said connectors, said network having conduits which are formed between said flexible films (65, 66) and which extend in said zone for treatment when said bag is clamped between said first shell and said second shell; said closed outline having a first side, a second side that is an opposite side to the first, a third side meeting the first and second sides and and a fourth side that is an opposite side to the third side and that meets the first and second sides (68, 69) and said connectors of the conveying network emerging to the exterior of a major part of said closed outline, inside and outside the first, second, and third sides (68, 69, 70); - first through apertures which are aligned and regularly spaced apart, two of the first through apertures being situated on respective opposite sides of the fourth side of said outline of the bag, and the other first through aperture being situated in the center of the fourth side of the bag, which first through apertures enable said bag to be positioned in said circuit; and - at least one second through aperture in the treatment zone of said liquid, between conduits of the conveying network, enabling the passage of means for clamping said bag in said circuit, said at least one second through aperture having a greater diameter than said first through apertures; - third through apertures, at least one of which being situated at the fourth side of said outline of the bag in the vicinity of the first through apertures situated at the top of the fourth side of the bag, at least another said third through aperture being situated at the opposite extreme, that is to say towards the bottom of the bag, in the treatment zone, said third through aperture serving for the positioning of a door of the installation in which is disposed said second shell; said first shell being disposed upright on a front face of a base and said second shell being mounted on said first shell; said bag being in said circuit and being clamped by said means for clamping; and said conduits being formed only when said bag is clamped. - two flexible films (65, 66) joined to each other by a seal delimiting a closed outline which form a zone for treatment of said liquid according to a closed outline, connectors for liquid emerging on the inside and on the outside of at least one side (68, 69, 70) of said outline and a network for conveying liquid between said connectors, said network having conduits which are formed between said flexible films (65, 66) and which extend in said zone for treatment when said bag is clamped between said first shell and said second shell; said closed outline having a first side, a second side that is an opposite side to the first, a third side meeting the first and second sides and and a fourth side that is an opposite side to the third side and that meets the first and second sides (68, 69) and said connectors of the conveying network emerging to the exterior of a major part of said closed outline, inside and outside the first, second, and third sides (68, 69, 70); - first through apertures which are aligned and regularly spaced apart, two of the first through apertures being situated on respective opposite sides of the fourth side of said outline of the bag, and the other first through aperture being situated in the center of the fourth side of the bag, which first through apertures enable said bag to be positioned in said circuit; and - at least one second through aperture in the treatment zone of said liquid, between conduits of the conveying network, enabling the passage of means for clamping said bag in said circuit, said at least one second through aperture having a greater diameter than said first through apertures; - third through apertures, at least one of which being situated at the fourth side of said outline of the bag in the vicinity of the first through apertures situated at the top of the fourth side of the bag, at least another said third through aperture being situated at the opposite extreme, that is to say towards the bottom of the bag, in the treatment zone, said third through aperture serving for the positioning of a door of the installation in which is disposed said second shell;