The glomerular basement membrane (GBM) regulates protein permeability by acting as both a size barrier and charge barrier. Since heparan sulfate-proteoglycan (HS-PG) which forms the charge barrier of the GBM contains a sulfonic acid base, we prepared negatively charged membranes by adding a sulfonic acid base to EVAL (ethylene vinyl alcohol) membranes. The permeabilities os the various proteins in these membranes were compared with those of conventional neutrally charged EVAL membranes. The permselectivity for proteins of negatively charged EVAL membranes depends largely upon the interrelation between the pore size of the membranes and size of the proteins. The negatively charged EVAL membranes had a higher selective permeability than the conventional neutrally charged EVAL membranes. Therefore, a negative charge within the dialyzer might produce higher selective permeability than conventional dialyzers.