The structure of the novel antibiotic reductiomycin, isolated from Streptomyces griseorubiginosus nov. sp. and exhibiting antiviral and antifungal activities, was clarified by X-ray diffraction studies. Since its structure could not be unequivocally determined from spectroscopic data alone, the direct method using MULTAN was employed. The solution was based on 310 reflexions with |E₀| ≥ 1.30, and the E map with the lowest RKarle (28.0) revealed 22 non-hydrogen atoms. All atoms were located via successive Fourier, difference Fourier, and least-squares methods. Refinement was carried out by the block-diagonal least-squares method with anisotropic temperature factors for non-hydrogen atoms, resulting in a final R value of 0.098. Oxygen and nitrogen atoms were identified from temperature factor behavior, and hydrogen atoms were found on a difference Fourier map. Reductiomycin has a unique chemical structure with a strong intramolecular hydrogen bond (O...O distance: 2.509 A). The molecule is approximately planar except for the acetoxy group (dihedral angle of 76.5° between the main plane and acetoxy group plane), and its absolute configuration was not determined. The crystal structure viewed along the c-axis shows an intermolecular contact O(5)[1]...C(7)[2] of 3.15 A.