Abstract: The invention provides a disposable syringe which can be used once only.The syringe comprises a shaft having a shaped appendix to which a plunger slidable within the syringe cylinder is sealingly connected. At that end at which the hypodermic needle is mounted the cylinder comprises a hole into which a hollow element having a shaped element integral therewith and projecting into the cylinder is inserted and locked. This shaped element becomes securely coupled to a portion shaped complementary to it provided at the adjacent free end of the plunger carried by the shaft to prevent the re-use of the syringe after the shaft has been lowered to firmly engage the shaped portion with the shaped element projecting from the cylinder since withdrawal of the shaft would cause disengagement of the plunger from the shaped appendix of the shaft.