Rat liver plasma membranes, enriched in blood-sinusoidal or bile-canalicular regions by differential andsucrose-gradient centrifugation, were further purified by partitioning in an aqueous polymer two-phasesystem. This method separates membranes according to differences in surface properties rather than size anddensity. A several-fold increase in the ratio of leucine aminopeptidase (a bile-canalicular marker) and5'-nucleotidase to asialo-orosomucoid binding (a blood-sinusoidal marker) was obtained in one fraction,whereas another fraction gave a 2-3-fold increase in ratio of blood-sinusoidal to bile-canalicular markers.Furthermore, the markers for both regions of the plasma membrane, as well as markers for Golgi membranesand lysosomes, showed a heterogeneous behaviour on counter-current distribution.