A mixing system has a container with flexible walls (1), a fluid-tight drive space (6) and a mixing device. The container with flexible walls (1) has at least one head plate (5). The fluid-tight drive space (6) is outside of the container interior (2), and is enclosed by the head plate (5) and a cap (7) attached to the head plate (5). The mixing device has a stirrer shaft (3), onto which stirrer elements (4, 4′) are attached. The stirrer shaft (3) is guided from the container interior (2) through an opening in the head plate (5) and the section of the stirrer shaft (3) in the drive space (6) is guided by bearings (9, 10). A seal (8) is fit into the opening of the head plate, and encloses the stirrer shaft (3) and seals the container interior (2) from the drive space (6).