An amperometric immunosensor for fast immunoassay of protein A has been developed. The disposable immunoelectrode consists of highly dispersed carbon modified by immobilized anti-protein A antibodies. The flow-through technique permits high automation of the entire measuring procedure. The detection is based on peroxidase-labeled anti-IgG antibodies captured by the immunoelectrode as a result of immunointeraction. The immunoassay employing a 'sandwich' scheme for the determination of different proteins with a detection limit in the nanomolar range is demonstrated. The immunosensor allows fast determination of protein A with an overall analysis time as low as 22 min. It can be further developed forStaphylococcus aureusassay based on the protein A expressed by this microorganism. The immunoassay system can be arranged as a portable device for fast analyses. The environmental application of the device is associated with on-site monitoring of bacterial contamination under field conditions.