Methods for purifying a polypeptide from a composition comprising the polypeptide and at least one contaminant are described, which methods comprise the sequential steps of: (a) passing the composition through an ion exchange membrane, where the polypeptide and the membrane have opposite charge, at operating conditions comprised of a buffer having a pH sufficiently distinct from the pI of the polypeptide to enhance the charge of the polypeptide and a low ionic strength effective to prevent the shielding of charges by buffer ions, which cause the membrane to bind the polypeptide and the at least one contaminant, and (b) recovering the purified polypeptide from the effluent.