After reaction in a biotechnological process for the production of enantiomerically pure amino acids, the used biocatalyst, consisting of cells, cell debris and enzymes, must be separated. Several unit operations can be applied, e.g. filtration, centrifugation and ultrafiltration. On a production scale at DSM, filtration was used to separate the biocatalyst, but its performance was not satisfactory. Therefore, research was carried out in order to improve filtration and to develop an alternative separation process, in this case ultrafiltration, In filtration the use of a body feed of filter aid is required to avoid formation of a slimy layer of cells and cell debris on the filter cloth, which would cause a dramatic decrease in the filtration rate. A precoat of filter aid proved not to be necessary. The porous cake formed by gradual deposition of filter aid on the filter favours a high filtration rate and retention of biomaterials. The best results were obtained with a high grade of perlite, at a concentration of 10 to 15 kg/m(3) for a biocatalyst concentration of 4.3 kg/m(3). The observed average filtration rate for a batch time of 80 min and at a pressure of 2.5 bar was 580 kg/m(2) h. The filtrate was not entirely clear : the protein retention was 50%. This means that the proteins remaining in the filtrate must be removed in another process step, e.g, carbon adsorption. By means of ultrafiltration, a very clear permeate was obtained as all the cells, cell debris and enzymes were retained by the membrane (molecular weight cut-off 25 000 Da). The highest flux was observed at a high cross-flow velocity, 3.9 m/s. At low transmembrane pressure, 3.4 bar, the flux had approximately the same value as at 10.7 bar. From our experiments, it can be concluded that once a fouling or gel layer is formed, it is stable and it will not be removed by increasing the cross-flow velocity, lowering the pressure, etc. At a concentration factor of 10 and a batch time of 6 h the average flux observed was 25 kg/m(2) h. The protein retention was approximately 99%. The investment and operational costs are lower for ultrafiltration than for pressure filtration. However, due to a process alteration at a later date, neither an UF unit nor a filtration unit were required. The negative effects of the presence of enzymes and cell debris in the product solution were compensated by using carbon adsorption, which was already installed for the purification of the product.