The influence of bile on beta-galactosidase activity, cellular integrity, cellular retention of beta-galactosidase, and cellular permeability of five strains of Lactobacillus acidophilus was investigated. The five strains were also compared for bile tolerance. Two strains, 223 and 4356, were significantly less resistant to bile than the others (107, NCFM, and 606). beta-Galactosidase activity of all five strains was significantly higher in the presence of .3% oxgall than in its absence. Strain 107 showed the highest increase of enzyme activity in the presence of oxgall. Cells were not lysed in the presence of .3% oxgall, and beta-galactosidase was retained inside the cell even after extended incubation (60 min) in the presence of .3% oxgall. However, material that absorbed light at 260 nm leaked from the cells in the presence of oxgall. We concluded that, in the presence of bile, the permeability of cells of L. acidophilus increased, permitting more substrate to enter the cells, thus increasing the beta-galactosidase activity of whole cells.