The invention relates to a disposable injection syringe having a barrel (1) in which a piston plug (6) can be displaced by a piston rod (7). The piston rod (7) possesses a base (10) on which there is supported a locking disc (13) on advance of the piston and can thus be moved over a bead-shaped stop ring (4). On retraction of the piston rod (7) the locking disc (13) remains at the stop ring (4), however, and pulls the piston plug (6) off the piston rod (7). Renewed use of the syringe is reliably prevented thereby. <IMAGE>
