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Tho material Is subject to correction by the author. Permission to copy Is restricted to an abstract of not more than 300 words. ABSTRACTPurpose of this paper is to describe the results of an Italsider -CSM full scale research work carried out to determine the toughness level required for safety in gas lines in order to control the unstable ductile fracture propagation. According to the present point of view, the fracture propagation resistance of ductile material is mainly conditioned by its toughness properties (mostly evaluated through the Charpy impact test). The tests carried out by Battelle, B.G., A.I.S.I. and E.P.R.G. in order to correlate the Charpy-V test values with the ductile fracture behaviour covered a limited diameter range. Italsider and CSM extended the experiments beyond the diameters, thicknesses and material qualities previously considered: diameters up to 56" (1.422,4 mm), thicknesses up to 20 mm and control rolled and Q.T. steels have also been considered for the research; grades up to API x80 have been investigated. The paper will present: a) the general description of the burst testing station (basic layout and additional equipment) and the related control facilities; b) six bursting test results (testing operation and conditions, field data analysis); c) a comparison between the Battel e, B.G., A.I.S.I. and our test results; d) the use of crack-stoppers of the type experimented in the tests; e) the Q.T. pipes behaviour to compare the fracture propagation resist~nce of different metallographic structures in order to select the reli-_ able materials from the point of view of safety and economy.