Abstract: The syringe has a tubular body slidably housing a plunger connected to a needle, movable between a first, safe position and a second, working position, respectively retracted into and extended from the body, and a spring biasing the needle toward its retracted position. An expansible retaining member is attached to one end of the needle and overrides a step, defined on the tubular body, as the needle is moved from the first to the second position. An expander device is carried movably on the plunger and adapted to be received and held in a socket formed in the retaining member to expand it and hold the needle in the working position. An annular rib is provided on a lug connected to the plunger for dislodging the retaining member from the step and release the needle, which is returned by the spring to its first position, out of sight inside the tubular body.