Mutants of Saccharomyces cerevisiae, deficient in cytosolic superoxide dismutase and catalase activities were used to study the role of various oxygen species in the process of lipid peroxidation in yeast cells. Lipid peroxidation does not occur normally in yeast, because this organism is unable to form fatty acids with more than one double bond, whereas under physiological conditions, only fatty acids with at least two double bonds undergo this process. The fatty acid content of cellular lipids was modified by growing the cells in anoxia in the presence of oleic or linolenic acid. Toxic effects of oxygen were observed almost exclusively in those cells of yeast mutants deficient in superoxide dismutase, which contain linolenic acid in cellular lipids. Hypersensitivity of the mutant cells, however, results mainly from toxic effects of the products of autooxidation of extracellular fatty acids. These facts suggest that superoxide dismutases are in some way involved in preventing toxic effects of the products of lipid peroxidation and to some extent prevent the process of lipid peroxidation.