The aim of this study is the investigation of the separation characteristics of sodium fusidate by means of ultrafiltration. The influence of several parameters, such as feed concentration, applied transmembrane pressure, pH, ionic strength, temperature and membrane material, on the retention and flux behavior have been investigated. The experiments were performed in a thin channel module, with four different ultrafiltration membranes, in order to choose a membrane with a proper retention and good operating conditions. In addition, the influence of the hydraulic permeability of the membranes, taking into account each experiment and contact time with solutions of sodium fusidate and dextran, was also analyzed in this work. The film theory model was used to estimate the mass transfer coefficient by means of the variation velocity method. The membrane-sodium fusidate interactions were studied by measuring the water flux of the membranes and by using the finely pore model, which combines both the viscous flow and frictional models. The present study gives good insight on the interactions within membrane filtration in the downstream processing of sodium fusidate. However, the situation is often more complex in real fermentation broth, since fermentation media contain hydrophobic or surface active substances which strongly influence the performance of the membrane.