A new antibiotic with weak cytostatic activity, named dioxolamycin (due to containing a dioxolane ring), was isolated from the cultured broth of Streptomvices filamentosus MC521-C5 (a requinomycin-producing strain). The strain was shake-cultured at 27°C in a specified medium, and the antibiotic was purified through activated carbon column chromatography (eluted with 50% aqueous acetone), silica gel column chromatography (using ethyl acetate), and recrystallization from hot ethyl acetate to obtain colorless crystals. Dioxolamycin had no antibacterial activity at concentrations <100μg/ml, showed moderate in vitro cytostatic activity against L-1210 leukemia cells (IC50 32.5μg/ml), an LD50 >200 mg/kg (mice, iv), and no significant in vivo activity against L-1210 leukemia ascites tumor. Its physico-chemical properties were determined: mp 214-215°C; [α] -53.1° (c 0.36, MeOH); UV 218 nm (E₁%₁cm 316.5); IR (KBr) cm⁻¹ 3460, 3440, 3310, 2910, 2890, 1710, 1690, 1670, 1635, 1600; FD-MS m/z 258 (M-1); molecular formula C₁₁H₁₅NO₆ (deduced from elemental analysis and FD mass spectral data, with analytical values consistent with calculations); soluble in methanol, ethanol, acetone, ethyl acetate and chloroform, almost insoluble/insoluble in toluene, benzene and water. The molecular structure was solved by X-ray diffraction (orthorhombic P2₁2₁2₁ space group, R value 0.045) and NMR spectral data (¹H and ¹³C NMR shown in Table 1). The absolute configuration was determined as methyl [1R,5R,6R,8S]-8-carbamoyl-5-hydroxy-8-methyl-7,9-dioxabicyclo[4,3,0]non-2-ene-3-carboxylate via acid hydrolysis to (-)-4-epi-shikimic acid.