Abstract: Disposable syringe for injecting fluids having a tubular body (1), a working stem (5) coupled to a piston (4), with a clearance between them, whereby movements of stem (5) determine relative corresponding displacements, of a determined length, between stem (5) and piston (4). A built-in tubular auxiliary element (8) is attached to the piston (4) and stem (5) by a sliding friction toothed attachment, which, depending on such relative displacements, advances within the syringe in uniform increments unidirectionally. Predetermined displacement of element (8) causes the syringe to become unserviceable. Built-in configurations (9), (13) and (14) prevent movement between the stem and piston as well as withdrawing of these elements from the body (1) until use of the syringe.