The assembly comprises: an adaptor (2) for coupling with a medical container having a septum comprising: a gripping member capable of being mounted on the medical container neck a pierceable elastomeric piece (30) which in an inactive state of the adaptor is not in contact with pressure with the septum outer surface and which in an active state of the adaptor is intended to be in contact with pressure with the septum outer surface a blister comprising a shell (100) and a removable closing membrane which form a housing receiving the adaptor a retaining system which comprises first retaining means (26) arranged on the adaptor and second retaining means arranged on the shell the first retaining means being designed in a locked position of the retaining system to cooperate with the second retaining means as long as the adaptor is not in the active state so as to prevent the shell from being separated from the adaptor unlocking means (53) which are designed to place the retaining system in an unlocked position when the adaptor is in the active state in which unlocked position the first retaining means do not cooperate with the second retaining means thereby allowing removal of the shell.
