Sodium chloride enhanced oxygenase activity while curtailing carboxylase activity of Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase; EC 4.1.1.39) purified to electrophoretic homogeneity. Exposure to 200 mM NaCl brought about an increase in the potential of Rubisco to oxygenate RuBP by over 50%. On the other hand, proline suppressed both oxygenase as well as carboxylase activities of Rubisco. Interestingly, proline-induced suppression in oxygenase activity was significantly higher than that of carboxylase activity. Most amazingly, salt-stress-induced enhancement in oxygenase activity was fully alleviated by proline even when present at a concentration as low as 50 mM. The findings presented in this communication clearly demonstrate for the first time that stress-induced proline accumulation might have a critical role in lowering the loss in fixed carbon by curtailing salt-stress-promoted enhancement in oxygenase activity of Rubisco.