A method for assembling a mixer-container intended to receive a biopharmaceutical fluid (C) in order to mix it, wherein: - a container is provided, having a flexible wall defining an inner space suitable for filling with biopharmaceutical fluid (C), the container comprising: o a mixing device having a shaft having an adjustable length along a main axis (XX), the shaft extending rectilinearly between a lower end and an upper end , o a first bearing attached to the wall , the shaft extending at least into the inner space from the first bearing , - a rigid outer containment device of the container is provided, - a drive motor is provided, located outside the container , the motor being suitable for rotating the shaft of the mixing device , - the container is placed in the rigid outer containment device , the rigid outer containment device comprising a bottom wall and a peripheral wall defining a housing suitable for receiving the container , the flexible wall of the container being arranged on the bottom wall of the rigid outer containment device , and - knowing that the position of the motor is fixed relative to the rigid outer containment device and that the container is already placed in the rigid outer containment device , the length of the shaft along the main axis (XX) is adjusted by arranging the shaft facing the motor , on the side of the upper end , in order to enable the motor to rotate the shaft , the shaft extending vertically along the main axis (XX) with the lower end located towards a lower part of the container while the upper end is located towards an upper part of the container , being connected to the first bearing , and wherein the shaft has a first part and a second part which are movable in translation relative to one another along the main axis (XX), the first part comprising an element suitable for sliding in a rectilinear slot of the second part of the shaft by adjusting the length of the shaft , and wherein the first bearing is placed in the immediate vicinity of the motor , at a distance from the motor less than the length (If) of the rectilinear slot . - a container is provided, having a flexible wall defining an inner space suitable for filling with biopharmaceutical fluid (C), the container comprising: o a mixing device having a shaft having an adjustable length along a main axis (XX), the shaft extending rectilinearly between a lower end and an upper end , o a first bearing attached to the wall , the shaft extending at least into the inner space from the first bearing , o a mixing device having a shaft having an adjustable length along a main axis (XX), the shaft extending rectilinearly between a lower end and an upper end , o a first bearing attached to the wall , the shaft extending at least into the inner space from the first bearing , - a rigid outer containment device of the container is provided, - a drive motor is provided, located outside the container , the motor being suitable for rotating the shaft of the mixing device , - the container is placed in the rigid outer containment device , the rigid outer containment device comprising a bottom wall and a peripheral wall defining a housing suitable for receiving the container , the flexible wall of the container being arranged on the bottom wall of the rigid outer containment device , and - knowing that the position of the motor is fixed relative to the rigid outer containment device and that the container is already placed in the rigid outer containment device , the length of the shaft along the main axis (XX) is adjusted by arranging the shaft facing the motor , on the side of the upper end , in order to enable the motor to rotate the shaft , the shaft extending vertically along the main axis (XX) with the lower end located towards a lower part of the container while the upper end is located towards an upper part of the container , being connected to the first bearing , <EOS>