Novel weak amphoteric charged membranes having cysteine residues were prepared. Cysteine (Cys) is one of the amino acids and has an amphoteric ion pair, which consists of an amino group and a carboxyl group. The poly(ethylene glycol) derivatives, with cysteine residues added to their side chains (PEG-Cys), were graft-polymerized onto a porous cellulose acetate (CA) membrane. The zeta potential was measured using a streaming potential method at various pH values ranging from 2.5 to 10 to investigate the behavior of the charge groups on the pore surface of these membranes. The zeta potential of this PEG-Cys-grafted membrane was pH-dependent, and the zeta potential/pH profile showed a plateau which was characteristic of an amphoteric ion pair in the range pH 4/9. The apparent surface charge density on these membranes was obtained from the zeta potential. The experimental results could be well explained by a calculated value using a site dissociation model including the contribution of not only the amphoteric ion pairs on the grafted polymer chains but also the negative charge groups originating from the base CA membrane. Copyright 2001 Academic Press.