Waste rock from a South African gold mine was tumbled in a 600mm diameter mill. Factors influencing the rate of abrasion of the rock and the product size were examined, including rock mass, pulp hold-up, distribution of rock sizes, mill speed and lifter configuration. Tests were conducted on pebbles which had been rounded in the mill. It was demonstrated that abrasion rate per unit mass increases significantly with rock mass. A simple laboratory test for rock ＇hardness＇ was shown to be correlated to the rate of abrasion. The rate of production of finished product from the coarse rock compares well with conventional grinding under certain conditions. The implications of the findings for circuit design and control are discussed.