Abstract: A method is disclosed for the continuous manufacture under sterile or at least non-pyrogenous conditions of plastic bags provided with connecting nipples to be used for the storage of blood plasma or serum, milk and other vulnerable liquids.For the welding of connecting nipples to the outer wall of the bag a counter-electrode is employed and is inserted into the bag's interior. Normally the plastic bags are formed out of an extruded plastic tube after it has been flattened and coiled to a spool.The present method starts from a wider flat tube than that used in the prior art and which has a width equal to the length dimension of the bag to be made so the bag is open at its sides. A counter electrode formed as a sliding-block is located in the interior of the tube whose leading end is closed now. The sliding-block is displaced in a direction towards the spool and each time that the welding of the connecting nipples onto a given bag is completed, the sliding-block is passed to the next bag to be treated.