An amphoteric chitosan derivative, containing carboxymethyl (-COOH) groups and amine (-NH2) groups, had been prepared. FT-IR spectra confirmed the presence of gamma(sym)COO (1408 cm(-1)) and gamma(as)COO (1584 cm(-1)) on the structural units of the chitosan derivative, and C-13 NMR spectra revealed that carboxy methylation reactions took place not only on the -OH but also on the -NH2. N, O-carboxymethyl chitosan (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 a weaker positively charged characteristic at low pHs but a stronger negative charged characteristic at high pHs 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 membranes and RES 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 be suitable for resistant to protein fouling at high pHs or protein adsorption separations at low pHs applications.