Abstract: Disposable hypodermic syringe comprising a needle (1) which can be retracted and held inside the syringe which moreover comprises a syringe barrel (4, 5) with liquid chamber designed with a piston (14) with a piston rod (7). Opposite the piston (14) the liquid chamber has an entirely or partially yielding end wall (6) with an opening, and the needle (1) is in its end area opposite its point provided with a head (3) which is subjected to the pressure of a spring in the direction toward the yielding wall (6). The piston (14) has a projection with locking means (10) designed to interlock with the opening in the yielding wall (6) when the piston bottoms in that the needle simultaneously snaps into a cavity (9) in the piston through the opening in the yielding wall (6); further usage of the syringe is hereby prevented.