It is reported an application of a biosensor based technology employing Surface Plasmon Resonance (SPR) for studies of molecular interaction in real time. An inhibition immunoassay for the detection of the herbicide atrazine has been developed. The biospecific interface is a sensor chip to which an atrazine derivative has been covalently bound. Monoclonal antibodies (Mabs) against atrazine are mixed with the sample containing herbicide, then the solution reacted with the biospecific interface. As the interaction between free antibodies and the immobilized derivative of atrazine bound to the surface proceeds, the SPR response changes inversely related to the atrazine concentration. A detection limit of 0.05 ppb of atrazine in water can be reached; the concentration range is 0–1 ppb, the analysis time is 15 minutes. After each measure the sensor chip surface can be regenerated. The analysis has been performed with standard solutions (buffer) in distilled and tap water.