From the benzene extracts of culture filtrates and from cultivated mycelium of Irpex pachyodon (Pers) Quel., a wood basidiomycete with recently discovered antibiotic properties, we have isolated an antibiotically inactive, white crystalline compound with mp 89-91° and a pKa of 9.1. The present communication gives evidence of structure 1 (methyl acethydroxamyl acetate) for this metabolite. Elemental and mass spectral analyses established the molecular formula as C5H9NO4. The IR spectrum (KBr) exhibits bands at 3170, 1658, 1510 cm⁻¹ attributed to a secondary amide group and bands at 1750, 1240 cm⁻¹ assigned to an ester group. The NMR spectrum of the metabolite shows four signals (2.00 ppm s 3H, 3.82 ppm s 3H, 4.55 ppm s 2H, 9.73 ppm br 1H) of protons not coupled to each other. The peaks at 2.00 and 3.82 ppm indicate two methyl groups, of the secondary amide and of the methyl ester respectively. The peak at 4.55 ppm can be ascribed to a methylene group lying between -COOR and -OR' groups, and the broad band at 9.73 ppm can be assigned to a -NH- group. The Mass spectrum also supports these findings. All reported data suggest formula 1, but some uncertainty remains with respect to the hydroxamic oxygen atom, since the spectral data do not give unequivocal information on the presence of the -N-O-C- group, and another structure 2 cannot be excluded. The metabolite was synthesized by reaction of methyl bromoacetate with potassium acethydroxamate. It is known that this kind of reaction gives preferentially O-alkyl-hydroxamate, and this is supported by the fact that both natural and synthetic products do not react with ferric chloride as compound 2 would do. Moreover, when the metabolite was warmed with 6N HCl and the volatile products of hydrolysis were removed by steam during the reaction, methanol and acetic acid were found in the distillate. At the same time we obtained in good yields from the reaction liquor a product with mp 151-153° (dec.), identified as carboxymethoxylamine hemichloride by comparison with an authentic sample. Evidence was thereby obtained for the presence of the -N-O-C group and the above described chemical and spectral data confirm the structure 1. Studies on the biogenesis and the role of this rather unusual metabolite are at present in progress.