Spherical balls are the dominant grinding media used in ball mills. However, balls which are initially spherical, wear into non-spherical fragments. The proportion of worn, non-spherical balls in the charge of a mill fed with 50 mm balls is dominant in ball sizes less than 30 mm. Their effects on mill performance in terms of material breakage are not yet established. The variations of specific rate of breakage with single size feed and fractional filling U, were studied for the two media shapes (spherical and worn balls). Higher breakage rates were noted with spherical media than worn balls but the differences narrow with decreasing feed size and increasing material fractional filling, U. (c) 2005 Elsevier Ltd. All rights reserved.