In the course of screening for antiviral substances and interferon inducers from Actinomycetes, a new antiviral antibiotic, virantmycin, was isolated from the culture broth of strain AM-2722, a soil isolate identified as Streptomyces nitrosporeus. Seed culture and fermentation were conducted using a medium containing glucose, peptone, meat extract, dry yeast, NaCl, and CaCO3 (pH 7.0) at 27°C. The pure compound was obtained via ethyl acetate extraction, high performance liquid chromatography (silica gel, CHCl3-MeOH 30:1), and silica gel preparative thin-layer chromatography (benzene-acetone 2:1) as a white powder with a melting point of 42-43°C and a specific rotation [α]28D of -0.5° (c 1, CHCl3). Its molecular formula C19H26NO3Cl was established by elemental analysis and high resolution mass spectrum. The UV spectrum (in EtOH) showed maxima at 228 nm (ε 3500) and 306 nm (ε 8100), the IR spectrum (in CCl4) exhibited carboxylic acid characteristics (3200-2400 cm-1 and 1690 cm-1), and it was soluble in organic solvents (e.g., dimethyl sulfoxide, methanol, acetone) but insoluble in water. Antiviral activity tested by the plaque reduction test against eight animal viruses (VSV, SbV, WEE, NDV, Vac-DIE, Vac-IHD, HSV-1, HSV-2) revealed inhibition at very low minimum effective doses (0.003-0.04 μg/ml). Virantmycin also exhibited weak antifungal activity, with a minimum inhibitory concentration of 25 μg/ml against Saccharomyces sake, Piricularia oryzae, Trichophyton interdigitale, and Aspergillus niger. The LD50 value in mice (CDF1) by intraperitoneal injection was 5 mg/kg.