Ultra fine grinding of the plant tailings of a refractory silver ore was studied using a laboratory type vertical stirred media mill. Preliminary tests confirmed that ultra fine grinding substantially improves the extraction of silver from the tailings in cyanide leaching (i.e. 36% Ag extraction rate from the as-received tailings with d(80) of 100 mu m, c.f. 84% extraction rate after ultra fine grinding of the tailings with d80 of 1.2 mu m). In the ultra fine grinding tests, the effects of ball diameter (2-4.5 mm), stirring speed (200 800 r/min) and ball charge ratio (50% 80%) on the fineness of grind (d(80), mu m) were investigated through a Box-Behnken design. Increasing stirrer speed and ball charge ratio decreased fineness of grind while larger balls resulted in the coarser products. The tests demonstrated that a fineness of grind less than 5 mu m can be achieved under suitable conditions. Analysis of stress intensity indicated an optimum range of stress intensity of (0.8-2) x 10(-3) N.m for all power inputs.