The sensitivity of L. acidophilus to acid environments was dependent on the phase of growth(Fig. 2). Cells from a stationary culture inoculated into MRS5 at pH 3.0(zero time, To) proved to be naturally tolerant to acid stress but the acid resistance was rapidly lost as soon as the cells entered into the exponential growth phase. Maximal sensitivity was found at 0.06 g/l. As the pH of the culture decreased, the cells regained their resistance to acid shock and the survival capacity increased up to 4h of incubation followed by a plateau. During this period (between 4 and 6h) the pH fell to 5.1. Subsequently, the acid tolerance of the cells increased and was maximal after 10 h fermentation (early stationary phase).