The objective of this research project is to develop a rapid molecular method for monitoring nitrification in a wastewater reactor. In the developed method, a monoclonal antibody (Hyb 153-3) was used because it can specifically recognize non-denatured enzymes responsible for nitrite oxidation in Nitrobacter and Nitrospira. The recognition of these enzymes under non-denatured conditions can significantly simplify the procedures of future immunoassays for environmental samples collected from various natural and engineered systems. This paper presents the ability of the selected Hyb 153-3 antibody to recognize the non-denatured form of the nitrite-oxidizing enzyme of Nitrospira in an aqueous phase as well as when the enzyme has been immobilized on a solid surface. (c) 2005 Society of Chemical Industry