In this study, polysulfone-based cation-exchange membranes were successfully prepared through a sulfonation procedure and a membrane preparation process. By comparing the results from different conditions, the most favorable sulfonation condition is to react at 75 degreesC for 4 h, whereas the optimal preparation process is casting a 70 degreesC solution of 2 g sulfonated polysulfone in a 8 ml mixture of polyethylene glycol: 1-methyl-2-pyrrolidinone (v:v) = 5:3, exposing the film to an air of 50% relative humidity at room temperature for 3 min, and then immersing it into a 27 degreesC water bath. The prepared cation-exchange membranes (90 pin thick) had a water content of 0.68 and an ion-exchange capacity of 2.9 mueq/cm(2). In the batch adsorption at pH 7.4, the saturation capacity of cationic lysozyme was 0.0098 mumol/cm(2) and anionic BSA was not adsorbed. In the flow adsorption process with a 10-piece membrane stack in the plate-and-frame module, the results show that lysozyme and BSA were effectively separated and the lysozyme recovery was higher than 93%. The effects of inlet flow rate and operation mode were insignificant on the separation performance. In the last part of this paper, the isolation of lysozyme from hen egg white was conducted in a flow process with only one piece of the prepared cation-exchange membrane. A purification effectiveness of 20.7 and a lysozyme recovery of 51.1% were achieved at a flow rate of 10 ml/min. (C) 2004 Elsevier B.V. All rights reserved.