Phosphorylation-dephosphorylation reactions as a form of regulatory control are assuming increasing importance in our understanding of cellular processes. The phosphorylation-dephosphorylation reaction was the first example of covalent modification in control and has been found to be present in almost every type of cellular process including synthesis of cell constituents, metabolic pathways and even gene expression1. It has assumed increasing importance as a result of the finding that some proteins coded for by carcinogenic viruses apparently are involved in a phosphorylation reaction2,3. To better understand the principles underlying these processes, we have studied the role of phosphorylation of isocitrate dehydrogenase (IDH) in the regulation of the branch point between the tricarboxylic acid (TCA) cycle and the glyoxylate bypass. We report here that in the process of purifying the kinase and phosphatase, we made two surprising observations - The kinase and phosphatase activities appear to be associated with the same protein, and the phosphatase activity has an absolute requirement for ATP.