Permeation behavior of dextransulfates through negatively charged ultrafiltration membranes made from polyacrylonitrile-sodium styrenesulfonate copolymer was studied at various NaCl concentrations in the solution. The effect of electrostatic barrier in the negatively charged ultrafiltration membranes on the permeation was examined by using dextransulfates of two different molecular weights. At less than 0.02 M NaCl concentration, the restricted permeation resulting from electrostatic repulsion between the membrane and the solute was observed. The effect of the electrostatic interaction decreased with an increase in NaCl concentration. In addition, the permeation of the negatively charged membrane was compared with that of uncharged membrane having similar molecular weight cut-off characteristics.