Steers fed on a diet low in cobalt and copper developed cobalt deficiency in 6 to 10 months but failed to develop a copper deficiency although maintained for 19 months on a diet containing only 3 p.p.m. copper in the dry matter. It is suggested that the treatment of the cobalt deficiency condition with injected vitamin B12 or oral cobalt sulphate was a factor in preventing the development of a copper deficiency. The effect of the low dietary cobalt was first evident at a concentration of 0.044 to 0.049 mg/kg dry matter which is somewhat higher than some previously suggested acceptable minimum levels. Of various parameters monitored, loss of appetite and plasma glucose concentration were the best indicators of a developing cobalt deficient condition although plasma alkaline phosphatase was also useful on an individual basis.