An assembly comprising: • an adaptor for coupling with a medical container having a neck closed by a septum, said septum having an outer surface directed towards the outside of the medical container, the adaptor comprising: - a gripping member for securing the adaptor to the medical container, said gripping member being capable of being mounted on the neck of said medical container; - a pierceable elastomeric piece which, in an inactive state of the adaptor, is intended not to be in contact with pressure with the outer surface of the septum, and which, in an active state of the adaptor, is intended to be in contact with pressure with the outer surface of the septum - when said adaptor is placed on said medical container; • a blister comprising a shell and a removable closing membrane which form a housing receiving the adaptor with the adaptor in the inactive state, prior to the first use of the adaptor; characterised in that the assembly further comprises: • a retaining system which comprises first retaining means 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, 54) 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. • an adaptor for coupling with a medical container having a neck closed by a septum, said septum having an outer surface directed towards the outside of the medical container, the adaptor comprising: - a gripping member for securing the adaptor to the medical container, said gripping member being capable of being mounted on the neck of said medical container; - a pierceable elastomeric piece which, in an inactive state of the adaptor, is intended not to be in contact with pressure with the outer surface of the septum, and which, in an active state of the adaptor, is intended to be in contact with pressure with the outer surface of the septum - when said adaptor is placed on said medical container; - a gripping member for securing the adaptor to the medical container, said gripping member being capable of being mounted on the neck of said medical container; - a pierceable elastomeric piece which, in an inactive state of the adaptor, is intended not to be in contact with pressure with the outer surface of the septum, and which, in an active state of the adaptor, is intended to be in contact with pressure with the outer surface of the septum - when said adaptor is placed on said medical container; • a blister comprising a shell and a removable closing membrane which form a housing receiving the adaptor with the adaptor in the inactive state, prior to the first use of the adaptor; characterised in that the assembly further comprises: • a retaining system which comprises first retaining means 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, 54) 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. • a retaining system which comprises first retaining means 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, 54) 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.