Laboratory scale and pilot plant nanofiltration (NF) and reverse osmosis (RO) experiments with fermentation broths were performed with the following aims : (i) to quantify lactic acid rejection and to determine whether a theoretical model developed in a previous paper could be used to predict lactic acid rejection; and (ii ) to quantify fouling of NF membranes and to determine the major fouling mechanism. It was found that the rejection model developed, based on the extended Nernst-Planck equation, can be used to quantify lactic acid rejection of RO and NF membranes. Especially at high fluxes the prediction of lactic acid rejection using parameters determined with lactic acid/water mixtures was quite good. At low fluxes the predicted rejection of lactic acid was usually lower. Fouling of the membrane could be quantified in terms of three resistances : a membrane resistance, an initial fouling resistance, and a time-dependent fouling resistance. Empirical equations for the initial fouling resistances were developed and time-dependent fouling could be described either by a colloidal fouling model (ultrafiltered fermentation broth) or a gel layer model (fermentation broth). Evaluation of the three resistances by simulation of continuous and batch concentration experiments showed that during NF of an ultrafiltered fermentation broth the initial fouling resistance, resulting from concentration polarization effects. was the predominant resistance. For a fermentation broth the time-dependent fouling becomes more important than the initial fouling resistance. Protein fouling is the main cause of the time-dependent fouling. Therefore, it is recommended to remove proteins by ultrafiltration before NF.