Abstract: A safety syringe having a sheath advanceable into a covered position by operation of the plunger with a mechanism for securing the sheath in the covered position. A bar extends across the interior of the barrel. One or more arms depend from the sheath, engaging the plunger directly or indirectly. Depressing the plunger pushes the sheath, via the arms, into the covered position. The arms each include a first and a second tooth. As the sheath is advanced, the first tooth will encounter the bar. The leading edge of the first tooth is beveled to allow the tooth to pass over the bar. The trailing edge of the first tooth is not beveled, preventing the first tooth from being pulled back over the bar. The leading edge of the second tooth is likewise not beveled, also preventing the second tooth from passing over the bar. The first and second teeth lock the bar between them, preventing the sheath and arms from moving in either direction.