With the aim of developing a new type of immunoassay with electrochemical detection, a Nafion(R) film-modified electrode was used as an amplifying sensor to detect redox cationic labeled drugs by square-wave voltammetry, A series of amino drugs was labeled with three model redox labels, i.e. cationic cobaltocenium and procationic ferrocene and nitroxide groups. Several parameters were involved in the optimization of the analytical properties of the sensor, i.e. film processing and film thickness, accumulation time, ionic strength and ethanol content. The preconcentration of the labeled drugs was impeded by the competitive accumulation of hydrophobic amines. The accumulation process of the procationic labeled drugs was influenced significantly by the potential applied during accumulation, and their strong non-specific binding with proteins was observed in the presence of serum. Therefore the choice of cobaltocenium as a label was preferred. Optimized conditions were defined, which took into account these various aspects. The avidin-biotin system was proposed as an applicable extension of this approach.