Kinetic analysis demonstrated that the irreversible inhibition of nucleoside diphosphate kinase of Escherichia coli by desdanine proceeds via a reversible Enzyme-Inhibitor complex. It is known that pyruvate kinase of E. coli becomes inactive upon prolonged dialysis in the absence of a reducing reagent, such as dithiothreitol and that the inactive enzyme is reactivated if dithiothretiol is added. Desdanine inhibits this reactivation process. The effect is discussed in relation to inhibition in relation to the inhibition of growth of E. coli by desdanine under anaerobic conditions. Pyruvate kinase of E. coli changes not only in intracellular quantity but in its kinetic characteristics depending on growth conditions, aerobic or anaerobic. The enzyme shows a broad specificity for nucleside diphosphates, especially in the presence of AMP, and thus resembles closely nucleoside diphosphate kinase. The in vivo role of pyruvate kinase in supplying nucleoside triphosphates under anaerobic conditions is discussed.