Abstract: A single-use syringe has a piston assembly sealingly slidable within a bal. The piston assembly has a plunger (1) with a divergently tapered portion (4) and a resiliently deformable piston (2) with a through aperture (7), correspondingly tapered so that part of the tapered portion (4) may fit within the aperture, and a tapered skirt portion (6) adapted to surround a further part of the tapered portion (4). The through aperture is adapted to be plugged by the plunger. A locking ring (3) having a tapered aperture to surround said skirt portion is provided to hold the skirt (6) against the plunger end portion (4). When the plunger (1) is depressed within the barrel, the locking ring (3) is delayed by the wall of the barrel while the skirt (6) is pushed inwardly. Once the skirt (6) is unrestrained by the locking ring (3), outward movement of the plunger (1) will cause its end portion (4) to be pulled free from the piston (2 ). The piston (2) is thus non-retractable to prevent a second drawing-in of liquid.