A masterbatch composition comprising (percent by weight): A) 15%-50% of a polypropylene component having a melt flow rate of from 1 to 250 g/10 min.; and B) 50%-85% of an olefin polymer partially soluble in xylene at room temperature, containing from 60% to 85% by weight of ethylene; said masterbatch composition having (i) a value of the intrinsic viscosity [.eta.] of the fraction soluble in xylene at room temperature ([.eta.].sub.sol) equal to or lower than 2.9 dl/g, and (ii) a value of the ratio MFR/[.eta.], , , , equal to or lower than 4, is added to polyolefin materials to obtain final compositions suitable for injection molding.