Removal of antibiotics by reverse osmosis/nanofiltration (RO/NF) from a model wastewater of a manufacturing plant producing pharmaceuticals for veterinary use was investigated. The rejection of the examined antibiotics by the selected RO and the tight NF membranes is acceptably high, exceeding in most cases 98.5%. The loose NF membrane retains the smaller antibiotics molecules less effective. Relating the solute rejections to the membranes' porosity has shown that the prevailing rejection mechanism of the examined antibiotics by all the membranes was the size exclusion effect. The rejection of the low molecular organic compounds by all the examined membranes is lower, ranging from 0.517 to 0.976 for the RO and tight NF membranes, and from 0.247 to 0.506 for the loose NF membrane. The rejections also follow the order of the solutes' molecular size. However, the specific physicochemical effects can influence the rejection of some low molecular organics by the examined membranes. (c) 2006 Elsevier B.V. All rights reserved.