Abstract: A hypodermic syringe (10) has a retractable needle (18) carried in A hypodermic syringe (10) has a retractable needsyringe body (11). The needle (18) is attached to a needle holder (22) which has a conical head (26) on its forward end. A passage (30) in the holder enables the needle bore to communicate through holes (29) in the holder (22) with the chamber (14). The plunger and piston assembly (32) has a piston (33) with a sealing cap (35) engaged therewith. The piston (33) has a forwardly extending tubular portion (39) defining an opening with a tapered wall (40) which engages over the head (26) to lock the head behind an abutment surface (41). At the same time a diaphragm (38) formed by the sealing cap (35) extending across the tubular portion (39) is ruptured so the chamber (14) is no longer sealingly enclosed.