Cellular activity of seed cultures of lactic acid bacteria is usually quantified in an indirect way by means of growth and production rates observed after transfer into a fresh culture medium. However, direct evaluation of the activity seems more suitable. Since inhibition of growth by undissociated lactic acid accumulated in the broth is now undoubtedly proven, the best performance can be expected from cells able to grow at a fast rate in the presence of high undissociated lactic acid concentrations. From this, the following criterion was proposed in order to evaluate cellular activity : the target function to be maximized is the product of the specific growth rate and the undissociated acid concentration. Such a criterion was used to optimize the medium composition, seed culture duration, and pH of precultures. In all cases, it was experimentally verified that the best volumetric productivity was obtained, after transfer to fresh medium, from cells corresponding to the highest value for the target function.