Assembly (100) comprising: - a connection element (20) capable of being mounted on a first neck defining an opening of a first medical container (1a), said first neck having an outer diameter D1, said connection element having a axial through hole (24) for accessing said opening of said first medical container, said connecting element having an outer diameter D2, with D1 strictly smaller than D2, - an adapter (10) comprising: or an element of (30) grip capable of being mounted laterally in said connecting element or in a second neck defining an opening of a second medical container (1b), said second neck having an outer diameter D2, said holding element having a central hole (34) for accessing said opening of said second medical container, or a perforable elastomeric piece (40), fixed with respect to said gripping element (30), arranged so as to face said hole (24) p central hole or hole (34) when said clamping element is mounted on said connecting element or on said second medical container, characterized in that - said connecting element (20) can be mounted axially on said first neck defining an opening of a first medical container (1a), the assembly (100) further comprises: - a protective envelope (50) comprising a rigid lining (51) substantially surrounding said connecting element (20) and said adapter (10) in a position of storage of the assembly, in which the connection element is separated from the adapter, said rigid container being closed by a removable film (52), - wherein said assembly further comprises temporary fixing means (27； 55) to prevent said connecting element (20) from escaping from said rigid container (51) when said film (52) is removed.Montaje (100) que comprende: - un elemento (20) de conexión capaz de montarse en un primer cuello que define una abertura de un primer recipiente (1a) médico, teniendo dicho primer cuello un diámetro D1 exterior, teniendo dicho elemento de conexión un orificio (24) pasante axial para acceder a dicha abertura de
