This work is focused on the first unit operation of the downstream process for the production of biological lactic acid: the clarification of the fermentation broth by cross-Row microfiltration. Different operating modes of microfiltration were compared in terms of productivity and fouling rate. The expected production could not he obtained during microfiltration with excessively high constant permeate flux values because the transmembrane pressure increase was too high. The main advantage of the constant permeate flux mode was the low fouling rate when it was operated below the critical flux. This fouling was due to adsorption of some molasses component and was must difficult to remove. The constant transmembrane pressure mode was more convenient because of higher productivity levels and moderate fouling rates.