Abstract: A hypodermic syringe assembly is provided with a syringe barrel and a safety shield telescoped over the syringe barrel and movable from a proximal position to a distal position. The safety shield is irreversibly lockable in the distal position on the syringe barrel to protectively shield a used needle cannula. The safety shield is releasably retained in a proximal position by engagement of a protrusion on the barrel and a groove on the shield. Engagement between the protrusion and the stop block can be overcome by rotating the safety shield. The safety shield may then be moved into an irreversible shielding position surrounding the needle cannula. The releasable retention of the safety shield in its proximal position prevents inadvertent distal movement of the safety shield.