When desalted extracts of soluble protein from dark-adapted wheat leaves were assayed for ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) activase activity in the presence of 1 mM ATP and an ATP-regenerating system, very little ATP-dependent activation of RuBP-inactivated Rubisco was found. In extracts from light-adapted leaves a very similar pattern of Rubisco activation was observed except that the overall level of Rubisco activity was much lower than in the extracts from dark-adapted leaves. These features were apparent both at low (120μg per ml) and high (640 μg per ml) protein concentrations. We were unable to demonstrate Rubisco activase activity in crude leaf extracts. Consequently, in order to establish that Rubisco activase was present in wheat leaf extracts the wheat leaf protein was purified to homogeneity. The identity of the protein was confirmed with antibodies to the spinach enzyme, ATPase activity and activase-mediated release of the inhibitor, carboxyara-binitol-1-phosphate (CA1P) from the tertiary Rubisco complex. The pure wheat Rubisco activase relieved the CA1P-induced inhibition of Rubisco activity. Rubisco activase had no significant effect on the affinity of wheat Rubisco for the substrate, ribulose-1, 5-bisphosphate (RuBP).