We have screened microorganisms isolated from soil and plants for their ability to produce antitumor antibiotics and isolated a novel antitumor antibiotic DC107 with the molecular formula C22H26N2O6S3 from a culture broth of a Streptomycete. In this communication, we report the production, isolation and characterization of DC107. The producing organism was isolated from soil collected in Natori-shi, Miyagi, Japan and taxonomically classified as Streptomyces sp. The seed medium contained glucose 10 g, soluble starch 10 g, Bacto-tryptone 5 g, yeast extract 5 g, beef extract 3 g, and CaCO3 2 g per liter (pH 7.2 prior to sterilization), which was inoculated with a stock culture and incubated for 48 hours at 28°C. The vegetative seed culture (0.9 liter) was used to inoculate a 30-liter jar fermentor containing 18 liters of medium consisting of soluble starch 50 g, soybean meal 20 g, CaCO3 5 g, KH2PO4 5 g, MgSO4·7H2O 0.5 g, and antiform agents LG 109 (Asahi Denka Kogyo) and KM-70 (Shinetsu Kagaku) per liter (pH 7.0 prior to sterilization), stirred at 300 rpm with aeration at 18 liters/minute at 28°C. Antibacterial activity was measured by the paper-disc method on nutrient agar using Bacillus subtilis as the test organism and usually reached a maximum after 2 days incubation at 28°C. The culture liquor was filtered, the filtrate adjusted to pH 4.0 with AcOH, and applied to a column of Diaion HP-20 (Mitsubishi Chemical Industries Limited). The column was washed with deionized water-MeOH-AcOH (5:5:0.02) and eluted with MeOH-AcOH (10:0.02). Active fractions were combined, diluted with equal volume of deionized water, and applied to a column of Diaion HP20ss, which was washed with deionized water-MeOH-AcOH (5:5:0.02) and eluted with deionized water-MeOH-AcOH (2:8:0.02). The active eluate was concentrated, extracted with EtOAc at pH 4.0, concentrated to dryness, and further purified by two stages of silica gel chromatography using hexane-EtOAc-AcOH (5:5:0.1) and CHCl3-MeOH (50:1) as eluents to give a crude precipitate, which was recrystallized from CHCl3 to yield 50 mg of white-needled crystals of DC107. DC107's physico-chemical properties include: MP 155°C (dec); readily soluble in MeOH, EtOAc, CHCl3, DMSO but insoluble in H2O and n-hexane; UV λmax nm (ε) 211 (14,000), 322 (12,000); [α]D5 -140° (c 0.1, MeOH); elemental analysis (calcd for C22H26N2O6S3·EtOAc: C 52.06, H 5.59, N 4.86, S 16.68; found: C 51.93, H 5.39, N 4.75, S 16.96); high-resolution fast atom bombardment mass spectroscopy (HRFAB-MS) m/z 511.1004 (M++1) (calcd for C22H27N2O6S3: 511.1031); IR and NMR spectra (1H, 13C) were described; it gave a positive reaction to ninhydrin, panisidine, iodine-azide reagents, but was negative to Rydon-Smith. Comparing with sulfur-containing antitumor antibiotics such as esperamicins and calichemicins, DC107 is evidently different in UV, optical rotation, IR and NMR spectra, indicating it is a new class of antibiotics. However, structure determination of DC107 seems difficult by spectroscopic studies due to its unique structure different from known antibiotics and instability to chemical degradation reactions; effort on structure determination by X-ray analysis is in progress and will be published elsewhere. DC107 exhibits a broad antimicrobial activity against Gram-positive and Gram-negative bacteria but not against fungi, and antitumor activity against murine leukemia P388 (ip, showing significant increase in life span (ILS 51%) at a dose of 0.38 mg/kg (ip)) and mouse sarcoma 18.0 (sc).