Lincoln Technology has developed a rapid biosensor-based assay called MICREDOX. While MICREDOX was originally developed for biochemical oxygen demand (BOD) monitoring, we now report on how this rapid assay has been extended to perform the direct toxicity assessment (DTA) of toxic chemicals on biological materials resident in the environment. The results show that microelectrode amperometry can adequately replace bulk electrolysis as a detection method. The correlation of these two methods is linear with a good fit (r2 = 0.97). Microelectrode amperometry facilitates the collection of time-course information and the development of toxicity profiles in analyte volumes less than one ml. The response of three bacteria (Bacillus subtilis B8, Escherichia coli K12 and Pseudomonas putida mt-2) to 3,5-dichlorophenol was examined using this revised methodology. EC50 values of 7.5 mg l-1 for B. subtilis, 7.0 mg l-1 for E. coli and 8.0 mg l-1 for P. putida were determined and are in line with EC50 values reported in the literature.