Water abstraction points may be affected by deliberate contamination. Early warning monitoring systems are required to alert operators to these contaminants and to changes in key water quality parameters. FastGuard is a result of a development programme to assess the suitability of the Fast Repetition Rate Fluorescence technology for detecting the effects of hostile contaminants. The ability of water borne algae to photosynthesize, and hence fluoresce, is significantly affected by toxic chemicals. FastGuard provides an instant response to a range of contaminants including methylparathion (an organophosphate insecticide), potassium cyanide (which can result in nervous system and respiratory failure), diuron and paraquat.