A prosthesis 100 is compressed in a device 10 by being wrapped in a flexible sheet 60 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 50 pulls the sheet 60 outwardly of the cartridge causing compression of the prosthesis. Stop projections 72, 73 are 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 prosthesis when compressed and a pusher rod 90, FIG. 4, is pushed through the tubes to push the prosthesis into an introducer sheath 36. The or each end piece 14, 16 may incorporate a ratchet mechanism 120 to prevent rotation in the wrong direction.
