The activation and properties of succinate dehydrogenase [EC 1.3.99.1] of the membrane vesicles of Escherichia coli were studied. Most of the enzyme activity was found to be latent in the membrane vesicles and was activated by various treatments of the membranes, such as preincubation with succinate, addition of Triton X-100, or freeze-thawing. Enzyme activation was achieved most effectively by preincubation of the membrane vesicles with succinate. Potassium cyanide enhanced the rate of activation by succinate. The activation achieved by treating the membranes with Triton X-100 or freeze-thawing them was instantaneous. Magnesium ions inhibited the activation of enzyme. The properties of succinate dehydrogenase of the membrane vesicles, including its Km values for succinate and phenazine methosulfate, and its pH optimum, are described.