Abstract: The syringe according to the invention comprises elastic means for blocking the intake stroke of the plunger which are initially located in an inactive position and are transferred into an active position during the first syringe intake and injection operation. When in said active position the elastic means are disposed in a position secured to the cylindrical body of the syringe and interfering with the intake stroke of the plunger.The syringe of the invention also comprises a slidable cap coaxial to the syringe body and covering the body itself, and being provided at its front with an exit hole for the needle and having members for its fastening to the syringe body, and elastic thrust means. The cap when in its retracted position leaves the needle exposed and when in its advanced position covers the needle, the fastening members being moved into their released position by elastic means on the first forward movement of the plunger.