There is described a closing plug (3) for a sealed sterile vial (1) containing medical or nutritional active substances, which is suitable for the sterile connection to a container of liquid diluent solution. There is also described a sterile connection system between a sealed sterile vial (1) closed by means of a plug (3) and a tube (10) for drawing said active substances, for mixing them with a liquid diluent solution inside a sterile container. The plug (3) requires an internal sterile chamber (5) in which, upon the piercing of an outer film (4) for sealing the plug (3), a compressible protection cap (12) of a piercing tip (11) of the tube (10) is inserted, which abuts against a seat (8) of the sterile chamber (5) allowing the tip (11) to pierce the end wall (13) of the protection cap (12) and then the inner membrane (6) of the closing plug (3). An intermediate pierceable membrane (15) placed on the seat (8) of the internal chamber (2) of the vial (1) may be included. (Fig. 1)
