To improve charged characteristics of chitosan (CS), we have prepared an amphoteric chitosan derivative containing carboxylic acid (-COOH) groups, i.e. NO-carboxymethyl chitosan (CM-CS). The CM-CS is soluble in water over a wide pH range but is almost insoluble at its isoelectric point (IEP, pH 5.5). CM-CS/poly(ethersulfone) (PES) composite microfiltration (MF) membranes were prepared by immersing PES MF membranes into CM-CS solutions and cross-linking with glutaraldehyde. Streaming potential measurements indicate that the CM-CS/PES composite MF membranes possess weaker positively charged characteristic at low pH but stronger negatively charged characteristic at high pH than the chitosan composite membrane (CS/PES). It was further observed that the CM-CS/PES composite membranes have higher adsorption capacities of bovine serum albumin (BSA) than the CS/PES composite membrane and PES membrane at lower pH 3.0-4.7, and lower adsorption capacities at higher pH 6.0-8.0. Therefore, the CM-CS/PES composite membranes may resist protein fouling at high pH applications, and separate protein by adsorption at low pH applications. (C) 2003 Elsevier Science B.V. All rights reserved.