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.
