Provided is a cassette unit suitable for use with an auto-injector having an electrically powered drive unit. The cassette unit comprises a cassette unit housing defining a housing cavity, a rearward entrance to the cavity and a forward needle projection aperture. The housing cavity may receive a syringe. Axially movable within the barrel of the syringe for forward movement into contact with the syringe plunger, there is a plunger slaving part defining a circumferential wall arranged for frictional sliding relationship with the inner wall of the barrel, a rear drive-receiving face and a front plunger-contacting face. In capping relationship with the rearward entrance of the cassette unit housing there is an end-cap defining a drive rod-receiving opening for receipt of a drive rod for providing forward axial drive to the plunger slaving part. In a pre-use configuration, the plunger slaving part is shaped for releasable engagement with the end-cap.