A brief history of Rubisco (ribulose bisphosphate carboxylase oxygenase) research and the events leading to the discovery and initial characterization of Rubisco activase are described. Key to the discovery was the chance isolation of a novel Arabidopsis photosynthesis mutant. The characteristics of the mutant suggested that activation of Rubisco was not a spontaneous process in vivo, but involved a heritable factor. The search for the putative factor by 2D electrophoresis identified two polypeptides, genetically linked to Rubisco activation, that were missing in chloroplasts from the mutant. An assay for the activity of these polypeptides, which were given the name Rubisco activase, was developed after realizing the importance of including ribulose bisphosphate (RuBP) in the assay. The requirement for ATP and the subsequent identification of activase as an ATPase came about fortuitously, the result of a RuBP preparation that was contaminated with adenine nucleotides. Finally, the ability of activase to relieve inhibition of the endogenous Rubisco inhibitor, 2-carboxyarabinitol 1-phosphate, provided an early indication of the mechanism by which activase regulates Rubisco.