This paper outlines the background, objectives, methodology, findings, outputs and recommendations from the Direct Toxicity Assessment (DTA) Demonstration Programme. This was a trial of a suite of bioassay methods and a seven-step protocol designed to deliver water quality improvements in catchments with well-defined water quality problems, where ecotoxicity from effluents was a contributing factor. The trial was run as a collaborative venture between the environmental regulators and water and manufacturing industries in the UK and was conducted at three project sites: -a reach of the river Aire near the city of Bradford in Yorkshire; -a reach of the River Esk near the town of Langholm on the border between Scotland and England; and -the lower Tees estuary on the north-east coast of England. The outcomes of each project are summarised in this paper. The learning points delivered by the programme were used to make recommendations to the regulators on how best to use bioassays for the assessment and control of complex effluents in the UK. Guidance was provided on how to carry out the bioassays and on how to use the data generated for regulatory decision-making. The programme also demonstrated how the regulators and the regulated can successfully work together to tackle environmental issues and deliver effective and workable solutions.