Barley plants grown in Mg, Mn, Zn and Cu deficient media were analyzed for specific activity, total activity and amount of RuDPCase present. Percent dry weight, as well as chlorophyll and soluble protein content were determined. All deficiencies caused lower chlorophyll and RuDPCase concentrations than occurred in the control. Soluble protein content, however, was higher in all deficient plants than in the control, either on a dry or a fresh weight basis. The addition of 18 μM Mn^2+ increased the activity of Mg^2+ containing RuDPCase preparations as much as 133% above normal. The specific activity of RuDPCase from Mg, Mn, Zn and Cu deficient plants was 60%, 118%, 159% and 149%, respectively, of the control. After 20 hours preincubation with Mg^2+, RuDPCase from Mg deficient plants attained 96% of the specific activity exhibited by the control. Total activity of RuDPCase from Mg, Mn, Zn and Cu deficient plants was 22%, 51%, 102% and 12 1%, respectively, of the control. Possible roles of Mn^2+ and Cu^2+ in the activation of RuDPCase are discussed.