A prosthesis is compressed in a device by being wrapped in a flexible sheet inside a cartridge, the opposite edges of the sheet being led out through a longitudinal slit in the cartridge and attached respectively to the outer surface of the cartridge and the inner surface of a surrounding shell; subsequent relative rotation of the shell pulls the sheet outwardly of the cartridge causing compression of the prosthesis. Stop projections are provided to limit the rotation to less than one complete revolution. End pieces of the device have tubes aligned with the prosthesis when compressed, and a pusher rod is pushed through the tubes to push the prosthesis into an introducer sheath. The or each end piece may incorporate a ratchet mechanism to prevent rotation in the wrong direction.