Two classes of newly synthesized amphiphilic compounds, phenolic antioxidants ("phenolics") and N-oxides exert in vivo antioxidant effects on live S. cerevisiae cells. Both groups have low toxicity, phenolics being more toxic than N-oxides and compounds with a longer alkyl chain having higher toxicity than those with a shorter alkyl chain. Phenolic antioxidants protect yeast cells exposed to the superoxide producer paraquat and peroxyl generator tert-butylhydroperoxide better than N-oxides at 3-fold higher concentration. Both types of antioxidants enhance the survival of pro-oxidant-exposed cells of S. cerevisiae mutants deficient in cytosolic and/or mitochondrial superoxide dismutase and could be good compounds which mimic the role of superoxide dismutases. The results of measurement of antioxidant activity in an in vitro chemiluminescence test differ from the results obtained in vivo with S. cerevisiae superoxide dismutase mutants. In contrast to their action on live cells, phenolics are less effective than N-oxides in preventing lipid peroxidation of an emulsion of lipids isolated from S. cerevisiae membranes.