Escherichia coli B contains two superoxide dismutases which differ with respect to their localization within the cell, the nature of their prosthetic metals, their responses to changes in (p)O(2), and their functions. One of these enzymes, which was liberated from the cells by osmotic shock and which was therefore presumed to be localized in the periplasmic space, is an iron-containing superoxide dismutase. The amount of this iron enzyme did not vary in response to changes in (p)O(2) during growth. In contrast, the other superoxide dismutase was not solubilized by osmotic shock, was a mangano-protein, and was found in greater amounts in cells which had been grown at high (p)O(2). E. coli, which had low levels of the iron-enzyme and high levels of the mangano-enzyme, as a consequence of growth in iron-deficient aerated medium, was killed by exposure to an exogenous flux of O(2) (-) which was generated either photochemically or enzymatically. The addition of bovine superoxide dismutase to the suspending medium protected these cells against this stress. On the other hand, E. coli, which had high levels of the iron-enzyme and low levels of the mangano-enzyme, as a consequence of growth in iron-rich anaerobic medium, was resistant to exogeneous O(2) (-). On the basis of these and of previously reported results (4a, Yost, F. J. and I. Fridovich, J. Biol. Chem., 1973, in press), it appears that the iron superoxide dismutase, of the periplasmic space, serves as a defense against exogenous O(2) (-), whereas the mangano-superoxide dismutase, in the matrix of these cells, serves to counter the toxicity of endogenous O(2) (-).