Abstract: A syringe has a barrel 1 housing a plunger 8 linked to a piston incorporating a seal 7 by a coupling member 9. A flange 12 on the coupling member 9 limits the extent of movement of the plunger and piston assembly, in association with latches 16. As the plunger 8 is operated through a sequence of expelling air from the syringe, to draw in liquid into the syringe and then to eject the liquid out again through a needle connected to an outlet 5, camways 11 at the ends of the coupling member 9 operate with projecting keys in such a way that the coupling member 9 is caused to rotate in stages. Then after the plunger is withdrawn again (after ejection of the liquid) the projecting keys 10 on the part associated with the piston seal 7 will become detached from the relevant keyways 11 so that the syringe cannot be used a second time.