A medical treatment apparatus, and in particular an extra-corporeal blood treatment apparatus, has a socket unit for the connection of a plug unit belonging to a device for supplying a medical fluid for the medical treatment apparatus. A filling apparatus for filling a device for supplying a medical fluid has a socket unit for the connection of a plug unit of the device for supplying a medical fluid. The device has a plug unit having means for indicating two states of operation, while the socket unit of the treatment apparatus has means for destroying the means for indicating two states of operation. When the plug unit is connected to the socket unit, the means for indicating two states of operation are destroyed, to rule out the possibility of the device being re-used. Re-use of the device is ruled out by the fact that the device cannot be re-filled with fluid.