A 1.2m diameter mill was used to develop a batch test procedure for measurement of the rate of abrasion and breakage of full-size rocks. Balls (75 mm) were used to increase grinding rates and an axial jet of water was used to flush fines out of the mill, through 20 mm peripheral ports. The rate of production of fines, per unit mass of rock in the mill, was used for modelling purposes. This was called the 'specific rate'. The mass of individual rocks was measured before and after each test to determine the effect of rock size and to check the data obtained by water flushing. Previous test data from 0.6 m and 1.8 m diameter mills is also presented. It was found the specific rate remained relatively constant over a wide range of sizes, justifying the use of an average rate for the calculation of mill performance. It was concluded that the semi-batch test in a 1.2 m diameter mill (or a 1.8 m diameter mill) offered a realistic and convenient way of determining the specific rate of rocks under conditions approaching those of a full-scale mill. (C) 2004 Elsevier Ltd. All rights reserved.