A syringe comprises a main tubular body, the body being open at one end and receiving a hypodermic needle at the other end, and a plunger, one end of the plunger extending into the body, through the open end thereof, the plunger having a longitudinal passageway therein permitting air flow therethrough. A fluid-tight seal is formed between the outside surface of the plunger and the inside surface of the syringe body. An air permeable filter membrane extends across the first end of the plunger, in the passageway, whereby the body can fill with blood, causing the air in the body to pass through the membrane to the open end of the body. Upon contact with the blood, the membrane swells and seals, automatically stopping the flow of blood. A one way valve extending across the passageway allows the syringe to be used to aspirate in the absence of natural blood pressure.