Escherichia coli K-12 cell suspensions in buffer were exposed to ozone at a concentration of 600 ppm. Measurements were made of cell viability, glyceraldehyde-3-phosphate dehydrogenase, malate dehydrogenase, lactate dehydrogenase, glutathione disulfide reductase, nonprotein sulfhydryl and total sulfhydryl compounds. Cell viability was not affected when E. coli K-12 was exposed to ozone for less than 10 minutes. The most sensitive parameter was glyceraldehyde-3-phosphate dehydrogenase followed by nonprotein sulfhydryl and total sulfhydryl compounds. Effects on malate dehydrogenase, lactate dehydrogenase and glutathione disulfide reductase were negligible. Cell survival and induction of lipid oxidation were also determined using two strains of E. coli K-12 (rec A, deficient in DNA repair and wild-type). The extent of membrane lipid oxidation correlated with cell viability in a dose-dependent manner and the survival curves of both strains showed similar sensitivity to ozone. The data suggest that the sulfhydryl group in the membrane is the primary target of ozone attack. Rec A DNA repair system does not appear to play a role in ozone resistance.