We present a model of polarisation at an ultrafiltration membrane. The feed is an aqueous solution of a large polyelectrolyte (such as a protein) and small co- and counter-ions. Small ions can pass the membrane; the large ones cannot. This causes electrical effects in the polarisation layer and at the membrane interface. Equilibrium at the interface then yields a pressure drop. The relation between this pressure drop and the flux is similar to that seen in practice. So are the salt retention and the effects of protein concentration, salt concentration and pH (charge of the protein).