A tyrosinase (Tyr) biosensor was developed based on Fe3O4 magnetic nanoparticles (MNPs)-coated carbon nanotubes (CNTs) nanocomposite and further applied to detect the concentration of coliforms with flow injection assay (FIA) system. Negatively charged MNPs; were absorbed onto the surface of CNTs which were wrapped with cationic polyelectrolyte poly(dimethyldiallylammonium chloride) (PDDA). The Fe3O4 MNPs-coated CNTs nanocomposite was modified on the surface of the glassy carbon electrode (GCE), and Tyr was loaded on the modified electrode by glutaraldehyde. The immobilization matrix provided a good microenvironment for retaining the bioactivity of Tyr, and CNTs incorporated into the nanocomposite led to the improved electrochemical detection of phenol. The Tyr biosensor showed broad linear response of 10 x 10(-8)-3.9 x 10(-5) M low detection limit of 5.0 x 10(-9) M and high sensitivity of 516 mA/M for the., determination of phenol. Moreover, the biosensor integrated with a FIA system was used to monitor coliforms, represented by Escherichia coli (E. coli). The detection principle was based on determination of phenol which was produced by enzymatic reaction in the E coli solution. Under the optimal conditions, the current responses obtained in the FIA system were proportional to the concentration of bacteria ranging from 20 to 1 x 10(5) cfu/mL with detection limit of 10 cfu/mL and the overall assay time of about 4 h. The developed biosensor with the FIA system was well suited for quick and automatic clinical diagnostics and water quality analysis. Crown Copyright (C) 2008 Published by Elsevier Ltd. All rights reserved.