Pseudomonas syringae pv. syringae is a phytopathogenic bacterium that attacks a wide range of hosts, and many strains produce phytotoxins such as syringomycin or syringotoxin, though their total structures had not been established. In Japan, bacterial blight of lilac (Syringa vulgaris L.) was found, and a strain (SY12) isolated from lilac produced antifungal toxins that were similar to syringomycin and syringotoxin but different in amino acid compositions, thus termed syringostatins. This paper describes the isolation and partial characterization of syringostatins. P. syringae pv. syringae SY12 was cultured at 26.5°C for 5 days in 500ml Erlenmeyer flasks containing 200ml of medium (PDB broth with 0.4% casamino acid and 0.6% malt extract). As syringostatins are very labile to basic conditions, they were purified under neutral or acidic conditions: the supernatant of centrifuged broth was successively applied to XAD-7, Sephadex G-25, and Sephadex LH-20 columns to obtain a toxin mixture, which was finally purified by HPLC with reverse-phase columns (SSC-ODS-H-3201 and SSC-CN-4251-N), yielding syringostatins A to H. Two abundant peaks showed strong antifungal activity, with the major component (second peak) designated syringostatin A, and the first peak further purified into syringostatins B and C. Some isolated compounds proved toxic to plants. High-resolution FAB mass spectrum of syringostatin A (1) revealed an (M+H)+ ion at m/z 1179.5970 with isotope peaks indicating a chlorine atom, giving a molecular formula of C50H87ClN12O18 (calcd. for C50H88ClN12O18, 1179.6028). Amino acid analysis of its hydrolysate showed threonine, serine, 2,4-diaminobutanoic acid, and ornithine in a molar ratio of 1:1:2:1. NMR analysis (1H-NMR, 1H-1H COSY, 13C-1H COSY) suggested the presence of β-hydroxyaspartic acid, homoserine, a β-hydroxytetradecanoyl moiety, 2-amino-2-butenoic acid (dehydrothreonine), an O-acylated serine residue, and a chlorine-containing amino acid (2-amino-4-chloro-3-hydroxy-butanoic acid, termed γ-chlorothreonine). The ten residues' structures were clarified, and syringostatin A's amino acid components differed from those of syringomycin and syringotoxin. FAB mass spectra showed syringostatins B, C, E, and G had isotopic peak patterns indicating a chlorine atom (likely γ-chlorothreonine), while D and F had normal patterns (no chlorine), suggesting they are analogs of syringostatin A. Syringostatin A is closely related to syringomycin but distinct from it and syringotoxin. Thus, syringostatins are a third type of toxin produced by P. syringae pv. syringae. Studies to deduce their total structure, including component stereochemistry, are ongoing.