Purified RuBP carboxylase requires activation by reaction with CO₂ and a divalent metal ion. Mg²⁺ is the most effective metal ion, and is probably involved in activation in vivo. Ca²⁺ is reported not to be an activator. Several oxyanions, including phosphate esters, are effectors of activation of RuBP carboxylase by Co₂ and Mg²⁺. It is now shown that Ca²⁺ is an effective activator of RuBP carboxylase and that PO₄ ^3-, FBP, NADPH and 6-phosphogluconate are effectors of this activation. The ratio of oxygenase to carboxylase activity of enzyme activated with Ca²⁺ is similar to that for the enzyme activated with Mg²⁺.