A surfactin mono-methyl ester was isolated from the cell broth of Bacillus subtilis HSO121 by acid precipitation, methanol extraction and two-step chromatographic methods. The structure was analyzed by GC/MS, HPLC, ESI Q-TOF MS/MS and NMR. It indicates that this ester is a C-15-surfactin-O-methyl ester of which the fatty acid portions are composed of iso-C-15 and anteiso-C-15 beta-hydroxy fatty acids. Based on the critical micelle concentration (CMC) in phosphate buffered saline (PBS, pH 8.0) and the IC50 value on Hela cell lines, the C-15-surfactin-O-methyl ester has a stronger surface activity and lower antitumoral effect than C-15-surfactin does. It is found that the Glu residues of surfactin-like lipopeptides play a role in their anti-proliferative effects on Hela cells. The possible antitumoral activity of surfactin is discussed in relation to their structures. (C) 2009 Elsevier Ltd. All rights reserved.