A polymeric membrane ion-selective electrode for determination of heparin is described in this paper,protamine is incorporated into the organic membrane phase and functions as sensing element for selective recognition of heparin.The proposed membrane electrode exhibits high selectivity for heparin over lipophilic anions such as thiocyanide and salicylate.The potentiometric response to the concentration of heparin is linear in the range of 0.01-0.4 U/mL and a lower detection limit of 0.005 U/mL can be achieved.