Abstract: Escherichia coli B cells were immobilized in an agarose membrane in a flow cell (preparation details given). The cells were then exposed to a continuous-flow of water (0.6 ml/min for each cuvette). The absorbance was measured against a similarly prepared membrane with no bacterial cells at 200 nm. On exposure to a toxin the bacterial cells become stressed and the metabolic rate changes resulting in a change in their UV spectra. Sodium azide and Tween-20 were used to represent non-specific biocidal agents as toxins. The system exhibited a 90% response in 15 s to the introduction of a toxin. The sensor was stable for up to 11 h; with further development the system could be used for industrial toxicity monitoring.