Different aspects of stress response of Lactobacillus acidophilus were investigated. First, the sublethal and lethal levels of bile, heat, and NaCl stresses were determined. They were 0.05% and 0.5% (bile), 53 degreesC and 60 degreesC (heat), and 2% and 18% (NaCl), respectively. To evaluate the effect of each stress at log pha-se, log-phase cultures were challenged directly with the lethal level of each stress (control) and were compared to log-phase cultures that were pre-exposed to the sublethal level prior to the exposure at the lethal level (test). Some, if not most, of the cells were killed in the control cultures against each of the three stresses. However, in the test cultures, the number of cells that had survived increased significantly. It appears that L. acidophilus is capable of displaying adaptive response to stress. The adaptive response to one stress was also shown to provide cross-protection against different stresses tested. The effect of each stress on stationary-phase cultures was also investigated. In contrast to log-phase culture, stationary-phase culture was inherently resistant to stress.