SummaryTo conclude this paper, the most important points may be summarized as follows.Firstly, pollution by dairy effluents will have to be stopped, not only for reasons of good public relations, but also because of impending trouble for future offenders when recent legislation begins to take effect. Secondly, the best and cheapest way of reducing the size of the problem is to reduce losses in the dairy of milk solids of all sorts, especially by‐products of low cash value but high polluting power. Thirdly, direct disposal of the remaining effluent is always best, where possible, but the only method of doing this which can be unreservedly recommended is by a pipe‐line to the sea or to the sewer of a willing Local Authority. Direct disposal on land may be a tempting method financially, but is only satisfactory in exceptional places, and is always a potential source of complaints from neighbours. Fourthly, for most dairies some kind of effluent purification treatment is unavoidable. At small non‐manufacturing dairies, where sludge disposal is not too difficult, the chemical precipitation process followed by irrigation has the advantage of simplicity and low capital cost. If space is limited, single biological filtration can be used for the final stage instead of irrigation. Scarlett's process of aerobic fermentation followed by biological filtration seems more suitable for larger dairies. The activated sludge process is relatively compact and cheap, but is unsuitable for manufacturing dairies with fluctuating load.Of the various biological filtration systems, in our experience the method of choice, when the load is heavy and where a high quality effluent is essential, is undoubtedly the double‐alternating system. It is unfortunately also very expensive. The cost of filter beds, which is the largest item, can be reduced by using one of the high‐rate systems, but these usually involve a bigger sludge disposal problem and an inferior effluent.Finally, I would like to repeat a plea often made before. An effluent plant should be regarded as part of the normal dairy equipment, and should be cared for like the rest of the factory. It should not be thought of as a mere nuisance, or something to be fitted and forgotten. In our experience, the effluent plant that is best looked after gives the least trouble and the best results. This is not achieved by putting it in charge of the most incompetent odd‐job man in the factory, who is thought fit for nothing better