A medicament delivery device system for delivery of a medicament from a cartridge (10). The system comprises a first container (11) for storage of a first substance, a second container (12) received in the first container (11) for storage of a second substance, and valve means (68) for closing a distal end of the second container (12). The system further comprises a disposable cassette (102) and a reusable firing unit (200). The cassette (102) has a body (104) for receiving the cartridge (10), and a mixing element (148) for displacing the second substance into the first chamber (16) in a mixing stroke, to mix with the first substance to form the medicament. The firing unit (200) comprises a drive element (208) for expelling the medicament from the first chamber (16) in a delivery stroke and a trigger arrangement (240) for initiating the delivery stroke.