A prosthesis (100, figure 2) is compressed in a device 10 by being wrapped in a flexible sheet (60, figure 2) inside a cartridge 12, the opposite edges of the sheet being led out through a longitudinal slit 40 in the cartridge and attached respectively to the outer surface of cartridge 23 and the inner surface of a surrounding shell 50； subsequent relative rotation of the shell pulls the sheet outwardly of the cartridge causing compression of the prosthesis. Stop projections (72,73, figure 2) may be provided to limit the rotation to less than one complete revolution. End pieces 14,16 of the device have tubes 22,32 aligned with the compressed prosthesis and a pusher rod (90, figure 4) is pushed through the tubes to push the prosthesis into an introducer sheath 36. The end pieces may incorporate ratchet mechanisms (120, figures 5 & 7) to prevent rotation in the wrong direction.
