a closure system for a medical liquid container having a dispensing outlet surrounded by a flange over which is fitted a plastic cap . the cap is externally threaded and has an outwardly - projecting brim at its lower end overlying the flange . the brim is sealed to the flange along an annular zone having an inner boundary defining an annular line of weakness , and an internally - threaded breakaway ring is threadedly disposed on the cap . the breakaway ring includes a plurality of circumferentially - spaced depending projections for stressing the brim along portions of the annular line of weakness and for initiating and propagating the rupture of the brim as the breakaway ring is screwed downwardly upon the cap .