Saccharomyces cerevisiae var. ellipsoideus contained 6.5 times more superoxide dismutase and 2.3 times more catalase when grown under 100% O(2) than when grown anaerobically. Growth under oxygen caused equal increases in both the cyanide-sensitive and the cyanide-insensitive superoxide dismutases of this organism. Experience with other eukaryotes has shown that cyanide sensitivity is a property of the cupro-zinc superoxide dismutase of the cytosol, whereas cyanide insensitivity is a property of the corresponding mangani-enzyme found in mitochondria. Cu(2+), which has been shown to increase the radioresistance of yeast, also caused an increase of both of the superoxide dismutases of S. cerevisiae. Yeast which had been grown under 1 atm of O(2) were more resistant toward the lethal effects of 20 atm of O(2) than were yeast which had been grown in the absence of O(2). Escherichia coli K-12 his(-) responded to growth under 1 atm of O(2) by increasing its content of catalase and of peroxidase, but not of superoxide dismutase. This contrasts with E. coli B, which was previously shown to respond to O(2) by a striking increase in superoxide dismutase. E. coli K-12 his(-) did not gain resistance toward 20 atm of O(2) because of having been grown under 1 atm of O(2). Once again, this contrasts with the behavior of E. coli B. These data indicate that, in both prokaryotes and in eukaryotes, superoxide dismutase is an important component of the defenses against oxygen toxicity.