The application relates to taking a sample of liquid contained in a flexible closed container, for example, a bag for collecting urine or blood. The problem to be resolved relates to piercing the wall of the container with a non- cutting piece, whilst obtaining a precise and clean opening for accessing the liquid and preventing any unintended flow or spill of liquid. This problem is resolved according to the present invention by a flexible bag (13, 14) for liquid, particularly a urine or blood bag which is characterised in that it contains a device (9) for taking a sample of liquid from said flexible bag. The sampling device (9) comprises a circular opening (1), arranged so that this opening is accessible from a first side of the device. The device (9) further comprises a support structure (2) on its other side, said support structure (2) comprising at least one support element (3), whose function is to distribute forces, and which is positioned at a distance from the plane of said opening (1).
