This study was undertaken to investigate the possible application of b-glucosidase-producing lactic acid bacteria as a functional starter cultures to obtain the bioactive isoflavones, genistein and daidzein, in fermented soymilk. Four strains – Lactobacillus plantarum KFRI 00144, Lactobacillus delbrueckii subsp. lactis KFRI 01181, Bifidobacterium breve K-101 and Bifidobacterium thermophilum KFRI 00748 – among the 31 lactic acid bacteria tested for b-glucosidase activity using q-nitrophenyl-b-D-glucopyranoside as the substrate were selected. Acid development, viable populations, and quantification of isoflavones using HPLC were performed at 0, 24, and 48 h of incubation at 37 C. The significant bioconversion (P < 0.001) of the glucoside isoflavones into their bioactive aglycones in soymilk fermented with four b-glucosidase-producing strains, with an average 7.1-fold increase of aglycones (daidzein+ genistein) was observed. There appeared to be correlations between the level of growth and b-glucosidase activity of each strain, and the hydrolysis of conjugated isoflavones in soymilk fermentation. Lactobacillus sp. were able to readily proliferate in soymilk than Bifidobacterium sp. (P < 0.05) and therefore completed more rapidly the hydrolysis of glucoside isoflavones.