We have isolated a potent new antitumor antibiotic duocarmycin A from a culture broth of a Streptomycete. In this communication, we report the production, isolation, physico-chemical and biological properties of duocarmycin A (also known as DC88-A). The producing organism was isolated from a soil collected at the foot of Mt. Fuji in Shizuoka, Japan and was taxonomically classified as Streptomyces sp. DO-88 (FERM BP 1002). Duocarmycin A was isolated from the culture broth by the following steps: the combined culture filtrate and acetone extract of the mycelium was adjusted to pH 5.5 with sulfuric acid and applied to a column of Diaion HP-20, washed with water and 50% methanol, eluted with ethyl acetate, concentrated in vacuo and extracted with ethyl acetate, the extract was concentrated and the residue was subjected to silica gel chromatography using a mixture of toluene-acetone as an eluting solvent, the active fractions were combined and evaporated to dryness, the residue was rechromatographed on silica gel (Lichroprep Si 60, Merck) with toluene-acetone, and the crude product was subjected to HPLC (Wako gel-LC-ODS30K) with a gradient elution (50% aqueous methanol to methanol) to yield 3 mg of pure duocarmycin A. The physico-chemical properties of duocarmycin A are summarized as follows: appearance of yellow powder; melting point 147~148°C; specific rotation +282°(c0.1, MeOH); molecular weight 507.1624 (calcd for C26H25N3O8: 507.1639); UV absorption at 310 (18,000), 358 (28,000), 425 (sh,8,000) nm; IR peaks at 3600, 3450, 3300, 1740, 1684, 1630 cm⁻¹; ¹H and ¹³C NMR signals consistent with its structure; solubility in MeOH, CHCl3, EtOAc, Me2CO, DMSO (soluble), hexane, water (insoluble). From an analysis of the ¹H and ¹³C NMR spectra together with mass spectral evidence, duocarmycin A is a close structural analog of DC89-A1 but lacks the Cl present in DC89-A1. Duocarmycin A showed strong antimicrobial activity against Gram-positive bacteria (MICs under 0.01 µg/ml for Staphylococcus aureus ATCC 6538P, Enterococcus faecium ATCC 10541, Bacillus subtilis #10107), with MICs of 0.032 µg/ml for Klebsiella pneumoniae ATCC 10031, 4.2 µg/ml for Escherichia coli ATCC 26 and Salmonella typhi ATCC 9992, 42 µg/ml for Pseudomonas aeruginosa BinHft l, 1.0 µg/ml for Proteus vulgaris ATCC 6897 and Candida albicans ATCC 10231, 4.2 µg/ml for Shigella sonnei ATCC 9290. The single dose LD50 in ddY mice was 0.034 mg/kg with iv administration. Duocarmycin A was effective against murine lymphocytic leukemia P388 transplanted in CDF1 mice (significant increase in life span) and showed strong antitumor activity against murine sarcoma 180 in ddY mice (T/C=0.35 at single ip dose of 0.04 mg/kg, T/C=0.26~0.42 at single iv dose of 0.03-0.0075 mg/kg). The results of our work show that duocarmycin A is a new antibiotic with high antimicrobial and antitumor potency.