A hypodermic syringe of the automatic or self-aspirating type for use in combination with disposable cartridge ampoules closed at the lower end by a diaphragm pierceable by a double-ended hypodermic needle and closed at the upper end by a slidable piston comprises a syringe holder adapted to generate aspirating conditions within the cartridge ampoule by the slight backward displacement of the slidable piston by means of a double plunger, one slidable within and biased against the other, the inner plunger being positively interengaged with the slidable piston of the cartridge ampoule, and axial movement of the outer plunger being restricted by frictional engagement with a collet in the end of the syringe holder.