Abstract: A disposable hypodermic syringe, comprising a tube, with its front part narrowed and adapted to receive a hypodermic needle, and a hollow piston capable of sliding through the inside of the tube; obturating devices laid across and held in operative position by the head of the needle; a spring, that tends to push the needle towards the inside of the tube; a closing device of the piston's cavity; and irreversible joining devices to fit other complementary joining devices placed at the head of the needle. The obturating devices are composed of a discoid part made of a slightly flexible material, having a central countersunk hole to receive the head of the needle that passes through it. This discoid part has, on its front side, appendages adapted to operate on the retention devices as the discoid part is pushed forward by the front end of the piston, when it completes its operative run.