β-mannanase from Trichoderma reesei was purified by DEAE Sephadex A-50 ion exchange chromatography and Sephadex G-100 gel chromatography. The molecular weight was determined to be 54 kDa by tricine SDS-PAGE. Trigonella foenum-graecum galactomannan was hydrolyzed by the purified β-mannanase, and then the hydrolysates were separated by Bio-Gel P2 chromatography. The main hydrolysates were composed of D.P. (degree of polymerization) 2, 3, and 4, 6 galactomanno-oligosaccharides. To investigate the effects of Trigonella foenum-graecum galactomanno- oligosaccharides on in vitro growth of L. plantarum, were cultivated individually on a modified-MRS medium containing carbon sources such as low- and high-molecular-weight galactomanno-oligosaccharide. Lactobacillus plantarum grew 1.8-fold after treatment with high- and low-molecular-weight galactomanno-oligosaccharides, compared to 1.3-fold for those with standard MRS medium. Bacteriocin was purified by Sephadex G-100 gel chromatography and determined to be 122 kDa by tricine SDS-PAGE. The bacteriocin activated doubly more effectively after treatment with galactosmanno-oligosaccharides compared to those with standard MRS medium. Bacteriocin showed antimicrobial activity against Staphylococcus aureus. The inhibitory compound lost activity when heated to temperatures greater than 30°C and when inhibited to pH changes that lowered the pH below 4 or raised it above 5. Furthermore, its effects were inhibited by treatment with proteolytic enzymes.