In the course of screening for new antibiotics, a novel nucleoside named oxanosine, which had antibacterial activity against Escherichia coli K-12 in a peptone medium, was isolated as crystals. The structure was determined to be 5-amino-3-~-D-ribofuranosyl-3H-imidazo[4,5-d][1,3]oxazin-7-one by X-ray crystallographic analysis. The oxanosine-producing strain was Streptomyces capreolus MG265-CF3 isolated from soil. The strain was precultured in a medium containing glucose 1.0%, glycerol 1.0%, sucrose 1.0%, oat meal 0.5%, soy bean meal (Prorich) 2.0%, pressed yeast (Oriental) 1.0%, Casamino acid (Difco) 0.5%, CaCO3 0.1% (pH 7.0 before sterilization) at 28°C for 2 days on a reciprocating shaking machine, then inoculated into a medium containing Bacto-Soytone (Difco) 1.0%, galactose 2.0%, corn steep liquor (Ajinomoto) 0.5%, dextrin 2.0%, (NH4)2SO4 0.2%, CaCO3 0.2%, antifoam 0.01% (pH 7.4 before sterilization) and cultured at 28°C for 5 days on a rotary shaking machine. The filtrate of the cultured broth was passed through a carbon column, the adsorbed material was eluted by acetone linear gradient, dried to get crude material, which was extracted with methanol. The methanol extract was purified by carbon chromatography and Avicel chromatography to obtain pure oxanosine crystals. The molecular formula of oxanosine was C10H12N4O6 (MW 284.23) confirmed by field desorption mass spectrometry and elemental analysis. Its UV spectra showed λmax in water (247 nm, logε 4.08; 288 nm, logε 3.93), in HCl (249 nm, logε 4.05; 288 nm, logε 3.91), in 1N NaOH (272 nm, logε 3.96). IR spectrum suggested sugar moiety (1000-1100 cm-1) and unusual chromophore (1795 and 1770 cm-1). 1H NMR (in D2O) showed 2-H δ8.45 (s), 1'-H δ6.33 (d, 6 Hz), 2'-H δ5.17 (dd, 6 and 6), 3'-H δ4.86 (dd, 6 and 4), 4'-H δ4.68 (m), 5'-H δ4.34 (m). 13C NMR chemical shifts (in DMSO-d6) were assigned in comparison with guanosine. Oxanosine showed weak antibacterial activity against Escherichia coli K-12 (MIC 12.5 mcg/ml), E. coli ML 1629 (25), Shigella flexneri 4b JS 11811 (6.25), S. dysenteriae JS 11910 (25), S. sonnei JS11746 (25), Proteus mirabilis IFM OM-9 (12.5), P. rettgeri GN 466 (12.5) and P. vulgaris OX-19 (25) on peptone agar, but no activity on nutrient agar at 100 mcg/ml. Guanine, guanosine and 5' guanylic acid had an antagonistic effect: guanosine reduced or eliminated its antibacterial activity. It inhibited HeLa cell growth in vitro (IC50 32 mcg/ml) and suppressed L-1210 leukemia growth in mice. Intravenous injection of 4 mg to mice (ca. 200 mg/kg) showed no toxicity.