Two strains of Actinomycete isolated from soil samples collected in Fujian and Jiangsu Provinces of China produce new antifungal antibiotics effective against the sheath blight disease of rice. These strains are both Streptomyces albofaciens. Two new antibiotics have been isolated in pure form and designated as albopeptins A and B. The actinomycetes were cultured at 28°C for 96 hr in a jar fermentor containing 18 liters of a medium of glucose (2%), soluble starch (1 %), meat extract (0.1%), dry yeast (0.4%), soybean flour (2.5%), NaCl (0.2%) and K2HPO4 (0.005%). The filtered broth (25 liters) was passed through a column of Diaion HP-10 (4 liters). The column was washed with 30% methanol and eluted with 50% acetone (4 liters), which was concentrated to 2 liters. The precipitates were collected by filtration, washed with water, and chromatographed on silica gel developed with butanol-methanol-water (8:1:1->4:1:1, stepwise). B-rich fractions were eluted first and then A-rich fractions appeared. The former fractions were collected and concentrated to dryness, giving 990mg of powder. This, dissolved in butanol-methanol-water (4:1:2) was further purified by Sephadex LH-20 chromatography, developing with the same solvent. A-rich fractions from the silica gel column were collected and purified in a similar way, giving 170mg of powder. Final purification was done by preparative HPLC [Nucleosil 5C18, isopropanol-methanol-water (2:2:6) plus 0.15% acetic acid, detection: UV at 220nm], giving pure albopeptins A (13mg) and B (42mg). Albopeptins A and B are amorphous white powders that decompose gradually above 235°C. They are freely soluble in formic acid and trifluoroacetic acid and sparingly soluble in acidic methanol and acetic acid, but hardly soluble in most other organic solvents and water. They showed a positive reaction to Rydon-Smith, iodine vapor, potassium permanganate, and anisaldehyde-sulfuric acid tests but were negative to the ninhydrin test. By paper electrophoresis, they remain at the origin over the pH range from 2 to 8. From elementary analysis and molecular weight measurement by FAB mass spectroscopy, the molecular formulas are C39H62N11O12S for Albopeptin A (FAB/MS: (M+H)+ m/z 924) and C40H65N11O12S for Albopeptin B (FAB/MS: (M+H)+ m/z 936). They are optically active: A, [α]D -90° (c 0.05, dimethylsulfoxide); B, [α]D -59° (c 0.03, dimethylsulfoxide). They showed end absorption in the UV region (λmax %HCl-MeOH 220nm shoulder). IR spectra of albopeptins A and B were similar and typical for peptides. From amino acid analysis of the hydrolysate (6N HCl, 110°C, 20hr) and 1H NMR spectra, Albopeptin A contains glycine (4), leucine (1), dehydrobutyrine (1); Albopeptin B contains glycine (3), valine (1), dehydrobutyrine (2). In addition, both A and B are considered to contain one mole each of a new sulfur-containing amino acid having a partial structure, -S-CH2-CH(NH)CO-, and a ω-substituted fatty acid moiety. Albopeptins A and B had a broad antimicrobial spectrum when assayed by an agar dilution method. A mixture of albopeptins A and B had preventive effect against sheath blight disease of rice plants, cucumber gray mold, and anthracnose in a pot test at the dose of 100~200 ppm. It also protected efficiently against blue mold disease of citrus fruit at 250 ppm. Mice tolerated 600 mg/kg when a mixture of A and B were administered orally.