Falconensones A and B are new type of yellow compound extracted from ascomycetous fungi, Emericella falconensis. Falconensone A p-bromophenylhydrazone and falconensone A dioxime are derivatives of falconensone A which have also been synthesized recently. The structural similarity of the falconensones to alpha-tocopherol (vitamin E) led us to investigate whether falconensones exhibit antioxidant activity. These studies found that falconensone A p-bromophenylhydrazone and falconensone A dioxime scavenge alpha,alpha-diphenyl-beta-picrylhydrazyl (DPPH) radical in a 1 : 1 ratio in contrast to vitamin E, where a 1 : 2 ratio relative to DPPH radicals was observed. In addition, linoleic acid peroxidation initiated by hydroxyl radicals was diminished by falconensone A p-bromophenylhydrazone to a greater extent than by vitamin E, and lipid peroxidation in rat liver microsome was reduced by falconensone A dioxime and falconensone A p-bromophenylhydrazone. In contrast, falconensone A and falconensone B, the 4'-nor-methyl derivative of falconensone A, showed much lower activity or were inactive in scavenging radicals. These results suggest that falconensone A p-bromophenylhydrazone and falconensone A dioxime, may be useful new antioxidant agents, wherein the bromophenyl and hydroxy residues of falconensone A may be important for antioxidant activity. Based on these results, derivatives of falconensone A appear to be effective antioxidants that may have clinical utility for diseases treated with vitamin E.