The present research addresses the question of whether Rubisco activase (R-A), the enzyme reported to activate Rubisco, is actually a molecular chaperone rather than a conventional enzyme. Several biochemical properties known to be characteristics of molecular chaperones were tested for R-A with positive results. The experiments were performed either in vitro with purified spinach Rubisco and Rubisco activase or in vivo in maize seedling leaves. Our results confirmed that activation of Rubisco by R-A is an ATP hydrolysis-dependent process and further demonstrated that (a) R-A binds preferably to non-native Rubisco protein, than to the native form, and dissociates from this complex after addition of ATP, (b) R-A increases during heat shock treatment in maize seedling leaves, and (c) a large recovery of Rubisco activity is achieved from heat-inactivated Rubisco by addition of R-A and an energy source. We conclude that R-A characteristics strongly suggest that this protein belongs to the molecular chaperone group. The possible role of R-A on maintaining Rubisco activity in vivo is discussed.