The detection of protamine, a polycation, by use of electrodes modified with self-assembled monolayers of thioctic acid is reported. These sensors can detect protamine concentrations as low as 0.5 μg/mL when [Ru(NH3)6]3+ is used as marker and 2.0 μg/mL when [Fe(CN)6]3- is used as marker. Binding of protamine to the thioctic acid monolayers controls the reduction rate of [Ru(NH3)6]3+ and [Fe(CN)6]3- at the electrode surface due to electrostatic attraction or repulsion between these markers and the monolayer, allowing the indirect detection of protamine. In physiological concentrations, the blood electrolytes sodium, potassium, calcium, and magnesium do not interfere. The sensors respond to protamine in diluted horse serum. They are selective for protamine over Polybrene, another polycation that neutralizes the anticoagulant activity of heparin. Protamine once bound to the electrode surface can be removed by washing with 0.1 M KCl of pH 5.1. An increase in the scan rate of cyclic voltammetry decreased the detection limit for protamine and increased the dynamic range. The sensor was used to detect the end point in heparin-protamine titrations.