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Oxygen induces superoxide dismutase in Streptococcus faecalis and in Escherichia coli B. S. faecalis grown under 20 atm of O2 had 16 times more of this enzyme than did anaerobically grown cells. In the case of E. coli, changing the conditions of growth from anaerobic to 5 atm of O2 caused a 25-fold increase in the level of superoxide dismutase. Induction of this enzyme was a response to O2 rather than to pressure, since 20 atm of N2 was without effect. Induction of superoxide dismutase was a rapid process, and half of the maximal level was reached within 90 min after N2-grown cells of S. faecalis were exposed to 20 atm of O2 at 37 C. S. faecalis did not contain perceptible levels of catalase under any of the growth conditions investigated by Stanier, Doudoroff, and Adelberg (23), and the concentration of catalase in E. coli was not affected by the presence of O2 during growth. S. faecalis, which had been grown under 100% O2 and which therefore contained an elevated level of superoxide dismutase, was more resistant of 46 atm of O2 than were cells which had been grown under N2. E. coli grown under N2 contained as much superoxide dismutase as did S. faecalis grown under 1 atm of O2. The E. coli which had been grown under N2 was as resistant to the deleterious effects of 50 atm of O2 as was S. faecalis which had been grown under 1 atm of O2. These results are consistent with the proposal that the peroxide radical is an important agent of the toxicity of oxygen and that superoxide dismutase may be a component of the systems which have been evolved to deal with this potential toxicity.