Abstract: A blood sampling unit having a barrel member consisting of a calibrated tubular body which is reduced at one end to form a projecting hollow neck for provision of a hollow needle containing a floating stopper and a slideable plunger. The plunger has an axially disposed gas channel in communication with the barrel and an orifice at its distal end. The unit is designed so that a supply of liquid or dry anticoagulant can be introduced into the portion of the projecting neck extending between the stopper and the needle. Alternately, the heparin or other anticoagulant can be obtained from a vial by drawing the heparin through the hollow needle and into the syringe proper. The plunger acts as a stop so that when its proximal end is set at the proper calibration for the desired sample, arterial blood pressure will move the floating stopper into abutment with the proximal end of the plunger while evacuating residual air through the axially disposed gas channel.