A medicament delivery device for obtaining and delivering a reconstituted medicament, said device comprising - a proximal housing part (12; 12"; 12"') adapted to accommodate a multi-chamber container (22; 22"; 22"') containing at least two agents; - a distal housing part (10; 10"; 10'") connected to said proximal housing part; - a medicament delivery drive unit accommodated in the distal housing part, wherein the proximal housing part and the distal housing part are configured to be movable relative to one another from an extended position to a retracted position whereby the medicament delivery drive unit acts on said multi-chamber container for mixing said at least two agents and thereby obtaining the reconstituted medicament; and - actuation means provided on the distal housing part and including an actuator (74; 74"; 74'") provided to engage the medicament delivery drive unit when operated by a user for delivering the reconstituted medicament; characterised in that - the medicament delivery drive unit is mounted in said distal housing part so as to be moved by the proximal housing part, when the proximal and distal housing parts are moved from the extended position to the retracted position, from a first position spaced from the actuator to a second position in which said medicament delivery drive unit can be engaged by the actuator, and wherein - the actuation means further include an activation member (74; 101; 201) which is displaceable between an inactive position wherein the actuator is prevented from engaging the medicament delivery drive unit and an active position wherein the actuator is able to engage the medicament delivery drive unit, whereby the actuator is capable of interacting with said medicament delivery drive unit to perform a delivery of the reconstituted medicament only in the active position of the activation member and in the second position of the medicament delivery drive unit, and wherein said medicament delivery drive unit comprises a spring-loaded plunger member (34; 34"; 34"') provided with a hollow space into which a drive spring (38; 38"; 38"') is arranged to spring-load the plunger, a drive spring holder (40; 40"; 40'") configured to partially surround the spring-loaded plunger member, a connection member (46; 46"; 46'") through which said drive spring holder is axially arranged, and an actuator member (70; 70"; 70'") being co-axially slidable on said connection member for holding said spring-loaded plunger member in a pre-loaded state. - a proximal housing part (12; 12"; 12"') adapted to accommodate a multi-chamber container (22; 22"; 22"') containing at least two agents; - a distal housing part (10; 10"; 10'") connected to said proximal housing part; - a medicament delivery drive unit accommodated in the distal housing part, wherein the proximal housing part and the distal housing part are configured to be movable relative to one another from an extended position to a retracted position whereby the medicament delivery drive unit acts on said multi-chamber container for mixing said at least two agents and thereby obtaining the reconstituted medicament; and - actuation means provided on the distal housing part and including an actuator (74; 74"; 74'") provided to engage the medicament delivery drive unit when operated by a user for delivering the reconstituted medicament; characterised in that - the medicament delivery drive unit is mounted in said distal housing part so as to be moved by the proximal housing part, when the proximal and distal housing parts are moved from the extended position to the retracted position, from a first position spaced from the actuator to a second position in which said medicament delivery drive unit can be engaged by the actuator, and wherein - the actuation means further include an activation member (74; 101; 201) which is displaceable between an inactive position wherein the actuator is prevented from engaging the medicament delivery drive unit and an active position wherein the actuator is able to engage the medicament delivery drive unit, whereby the actuator is capable of interacting with said medicament delivery drive unit to perform a delivery of the reconstituted medicament only in the active position of the activation member and in the second position of the medicament delivery drive unit, and wherein said medicament delivery drive unit comprises a spring-loaded plunger member (34; 34"; 34"') provided with a hollow space into which a drive spring (38; 38"; 38"') is arranged to spring-load the plunger, a drive spring holder (40; 40"; 40'") configured to partially surround the spring-loaded plunger member, a connection member (46; 46"; 46'") through which said drive spring holder is axially arranged, and an actuator member (70; 70"; 70'") being co-axially slidable on said connection member for holding said spring-loaded plunger member in a pre-loaded state.