In a syringe (100) comprising a syringe body (1) hollow on the inside, a plunger (4) slidable inside the syringe body (1) and provided at the rear with a manually operable shaft (41), and an injection needle (2) engageable to the front end (13) of the syringe body (1) by means of a needle-carrier (20), there is applied a set of components comprising a sleeve (5) mounted slidably over the syringe body (1) to pass from a retracted position of use of the syringe to a forward position of safety in which it covers the needle (2), spring means (7) disposed under compression between the sleeve (5) and the syringe body (1) and guide means (60) disposed in the sleeve and cooperating with complementary guide means (83) disposed on a guide element (80) made integral with the syringe body (1) to guide the axial movement of the sleeve (5) with respect to the syringe body.