Abstract: An assembly in-situ preparation of suspensions and solutions of an active, solid, product in a liquid product, each of the products being separately contained in a hermetic flask, with connecting means permitting the placing in communication of the two flasks, which means consists of a single piece (1) molded of plastic material with an upper portion (2) internally threaded for coupling with the neck of the flask (7) containing the liquid and a lower portion (3) capping the neck of the flask containing the solid product, the molded connecting piece is closed by a sealed elastomer stopper and an inner shoulder hooking over one or several small collars on the neck.