Abstract: An improved blood drawing syringe characterized by a barrel having a hollow cylindrical bore with an open rear end and a reduced diameter front end for the attachment of a needle, a pushrod disposed for slidable movement within the barrel, a piston attached to the forward end of the pushrod and having a circular disc with spaced apart forward and rear perforated faces enclosing a filter disposed between the faces, a sealing flange disposed around the periphery of the disc sized and dimensioned to fit slidingly within the cylindrical bore of the barrel and a flexible seal abutting the rear face of the disc and adapted to selectively cover the perforations in the rear face of the disc as a function of forward or rearward air pressure on the flexible seal.