Oxygen-derived free radicals have been known to play a role in a variety of diseases such as autoimmune disease, cardiovascular diseases, diabetes, inflammation, rheumatism and cancer-initiation1>2). Thus, it could be expected that free radical scavengers may contribute to the control of these diseases. In the course of our screening for free radical scavengers of microbial origin, we have isolated new active substances, naphterpin3), antiostatins4) and benthocyanin A5). Further screening resulted in the isolation of a new free radical scavenger, designated pyridoxatin, from a fungus culture identified as Acremonium sp. BX86. It inhibited lipid peroxidation induced by free radicals in rat liver microsomes free from vitamin E6). In this paper, we report the isolation and structure elucidation of pyridoxatin. Its molecular formula was determined as C15H21NO3 by HREI-MS and elemental analysis. NMR spectral analysis indicated that pyridoxatin existed as a mixture of two rotamers (approximately 1:1 in methanol-d4 and 2:1 in acetone-d6). The structure contained a 1-hydroxy-2-pyridone moiety and a 4,6-dimethyl-2-vinylcyclohexyl moiety, with the relative stereochemical structure depicted as shown in Fig. 4. The two rotamers were ascribed to the hindered rotation along the C-3～C-7 axis. Pyridoxatin was approximately twenty times (IC50 0.55 μg/ml) as active as vitamin E (IC50 10.8 μg/ml) in the assay system employed6). It inhibited hemolysis of rat erythrocytes catalyzed by 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH)9, a free radical initiator (IC50 1.95 μg/ml). Pyridoxatin inhibited the growth of HeLa cells at the concentration of 1.0 μg/ml (IC50), but did not show any antimicrobial activity except against Candida albicans (MIC 1.64 μg/ml).