<p>A pre-filled hypodermic syringe adapted for use with a retractable-type needle unit (18). The syringe has a barrel (10) provided with a piston member (14). The piston member (14) includes a plastics portion (30) mounting a dislodgeable blocking portion (36) of chemically inert material in such a way that the plastics portion (30) is not in contact with the pre-filled contents of the syringe.</p>
