The effects of sulphite ion (Formula) and sulphate ion (Formula) on both the activation and the catalytic activities of ribulose- 1, 5-bisphosphate carboxylase (EC 4.1.1.39) were studied and compared to those of other effectors of the enzyme, particularly inorganic phosphate (P1). The activation by CO2 and Mg2+ of a slow activating form of the carboxylase in the presence of the two anions produced high specific activities with significant lower concentrations of CO2 than normally required. This was due to stabilization of the ternary complex between the enzyme, CO2 and Mg2+. With a rapidly activating species of enzyme, Formula and Formula caused only a small increase in activation with subsaturating CO2.Formula, Formula and P1, with saturating concentrations of CO2 also enhanced the catalytic activity above that achieved with CO2 and Mg2+ alone; P1 was the most effective of the anions, producing a 50% increase in the specific activity, both with the slow and rapidly activating species. Formula and Formula were potent inhibitors of the carboxylase and oxygenase reactions of the enzyme. Formula was a non-competitive inhibitor with respect to CO2, and competitive/mixed with respect to ribulose-1, 5-bisphosphate. The time course of the carboxylase and oxygenase reactions in the presence of Formula were biphasic with inhibition apparent only in the second phase.