A novel cysteine-selective electrode based on lead phthalocyanine (PbPc) as ionophore is described. The electrode was prepared by incorporating PbPc into a plasticized poly(vinyl chloride) (PVC) membrane, which was directly coated on the surface of a graphite electrode. This electrode shows high selectivity for the response to cysteine, as compared with many common inorganic anions, salicylate, and other kinds of amino acids. The influence of membrane composition, pH, and the effect of lipophilic cationic and anionic additives on the response characteristics of the electrode were investigated. The resulting sensor demonstrates nernstian response over a wide linear range of cysteine concentration (1 x 10(-6) to 5 x 10(-2) M). The electrode has a fast response time, micromolar detection limit (approximately 1 x 10(-6) M), and good long-term stability (more than 1 month). The prepared electrode was used for determination of cysteine in a synthetic human serum sample, and very good recovery results were obtained over a wide concentration range of cysteine.