Abstract Extract At least under New Zealand conditions, the use of cobaltic oxide pellets (“cobalt bullets”) to control cobalt deficiency in sheep is limited by a tendency of the bullet to accumulate deposits of calcium phosphate (Andrews, 1961 Andrews, E.D. 1961. N.Z. Dept. Agric. Bull., 180: 23–23. [Google Scholar]). Although the use of steel “grinders” (for example, in the form of 1/2 in. × 1/2 in. engineer's grub screws) has been recommended as a means of keeping the surface of the bullet in a satisfactory condition (Anon., 1960 Anon Rur. Res. in C.S.I.R.O. 1960 34 10 10 [Google Scholar]), very little published information on the efficiency of the method is so far available from Australian sources, and, in one New Zealand experiment with lambs (Andrews, unpublished), grinders were of little effect in reducing the amount of calcium phosphate deposited.