A single use hyperdermic syringe (20) having a one-way valve (56, 62) to prevent refilling and a retractable needle (30). A needle connector (26) and a needle hub (28) with needle (30) form an assembly (40) with a cavity (52) for the passage of liquid. An elastically-deformable sealing ring (56) and a movable sealing member (62) in the cavity are configured to form a seal, after a single use of the syringe, against a flow of liquid into the syringe barrel (22). Locking pins (76) on the piston engage the needle connector when the barrel is emptied, whereby the needle is retractable into the barrel by moving the plunger (24) in the proximal direction.
