A masterbatch composition comprising, all percent amounts being by weight: A) 10-45% of a homopolymer or a copolymer of propylene; B) 10-30% of a copolymer of propylene with ethylene, containing from 18 to 45% of ethylene; C) 42-60% of a copolymer of propylene with ethylene, containing from 55 to 85% of ethylene; said composition having a MFR L value from 0.01 to 10 g/10 min., a value of the intrinsic viscosity [η]sol of the fraction soluble in xylene at room temperature from 1.5 to 2.5 dl/g and a value of the ratio [η]sol/MFR L equal to or lower than 6.