Abstract: A safety closing device for containers of biological liquids and, particularly, for test tubes used for the drawing, transport and/or analysis of blood. The safety closing device includes an undercap and a cap, both with central portions made of a perforable material. The central portion of the undercap is sealingly locked on the edge of the test tube and the central portion of the cap is sealingly locked on the side facing the undercap. A locking mechanism, that can only be intentionally disengaged, mechanically couples the cap to the undercap, and secures the cap/undercap assembly onto the prearranged open end of the test tube.