Isocitrate dehydrogenase kinase can use ATP but not other nucleoside triphosphates as a phosphate donor. It responds hyperbolically to both ATP and isocitrate dehydrogenase. The kinase is inhibited sigmoidally by low concentrations of DL-isocitrate and hyperbolically by ADP, AMP, NADPH, phosphoenolpyruvate and several other effectors. Isocitrate dehydrogenase phosphatase requires a nucleotide for activity; ADP and ATP are the best activators. The phosphatase responds hyperbolically to ADP or ATP, to Mg2+ ions and to phosphorylated isocitrate dehydrogenase. The phosphatase is activated twofold to threefold by AMP, oxaloacetate, pyruvate, phosphoenolpyruvate, 2-oxoglutarate and DL-isocitrate. It is inhibited hyperbolically by NADPH. The pH optima and the Km values for substrates of the kinase and the phosphatase are reported. We propose that the role of the phosphorylation of isocitrate dehydrogenase during growth of Escherichia coli on acetate is to render this enzyme rate-limiting in the citric acid cycle; this should cause an increase in the level of isocitrate and divert the flux of carbon through the glyoxylate bypass. We suggest that the phosphorylation state of isocitrate dehydrogenase in intact cells is controlled by the levels of isocitrate, phosphoenolpyruvate, NADPH and the adenine nucleotides. This theory can explain many recent observations on the control of the activity of isocitrate dehydrogenase.