The study determined beta-glucosidase activity of commercial probiotic organisms for hydrolysis of isoflavone to aglycones in fermenting soymilk. Soymilk made with soy protein isolate (SPI) was fermented with Lactobacillus acidophilus LAFTI L10, Bifidobacterium lactis LAFTI B94, and Lactobacillus casei LAFTI L26 at 37 degrees C for 48 h and the fermented soymilk was stored for 28 d at 4 degrees C. beta-Glucosidase activity of organisms was determined using rho-nitrophenyl beta-D-glucopyranoside as a substrate and the hydrolysis of isoflavone glycosides to aglycones by these organisms was carried out. The highest level of growth occurred at 12 h for L. casei L26, 24 h for B. lactis B94, and 36 h for L. acidophilus L10 during fermentation in soymilk. Survival after storage at 4 degrees C for 28 d was 20%, 15%, and 11% greater (P < 0.05) than initial cell counts, respectively. All the bacteria produced beta-glucosidase, which hydrolyzed isoflavone beta-glycosides to isoflavone aglycones. The decrease in the concentration of beta-glycosides and the increase in the concentration of aglycones were significant (P < 0.05) in the fermented soymilk. Increased isoflavone aglycone content in fermented soymilk is likely to improve the biological functionality of soymilk.