The construction, performance characteristics, and application of a polymeric membrane coated on a graphite electrode with unique selectivity toward histidine are reported. The electrode was prepared by incorporating chloro(5,10,15,20-tetraphenylporphyrinato) manganese(III) [Mn(TPP)Cl] into a plasticized poly(vinyl chloride) membrane. The influences of membrane composition, pH, and foreign ions were investigated. Calibration plots with near Nernstian slopes for histidine were observed, -55.4 mV/decade, over a linear range of four decades of concentration (1 × 10(-)(5) to 1 × 10(-)(1) M). The electrode has a detection limit of 5 × 10(-)(6) M histidine and shows a fast response time of about 1 min. The electrode shows high selectivities toward histidine over several amino acids and common inorganic anions.