Results presented here are from a study investigating biological nitrification inhibition in full-scale BNR plants. Nitrification inhibition was studied using a novel extant respirometric field assay that was developed as a rapid and robust technique for the biokinetic characterization of biomass from municipal and industrial wastewater treatment plants. Once developed, tested and optimized, the field assay was used to measure the inhibitory effects of various chemicals commonly found in industrial wastewater. These components included: heavy metals, surfactants, and metal complexing agents, and toxic inorganic anions. The inhibition study was carried out using activated sludge from a treatment plant that performs consistent nitrification and receives significant industrial wastewater flow. Results indicate that the newly developed field assay is capable of measuring even small degrees of chemical inhibition of nitrification. The assay shows great promise as a tool for plant operators, plant engineers, and consulting engineer with many applications including: 1) routine monitoring of nitrification activity 2) troubleshooting and sensitivity testing; 3) planning and implementing industrial pre-treatment programs; and 3) designing new and updated treatment plant processes.