Testing under laboratory conditions is undoubtedly useful in assessing the activity of disinfectants. However, such testing must be regarded as no more than a preliminary to field trials. This not only indicates the unreliability of laboratory tests, but also poses the wider problem of how any laboratory evaluation can be correlated with the field requirements. Many attempts have therefore been made to devise tests which are suitable to evaluate the in-use requirements of disinfection (e.g. on farms). The aim of these techniques is to record the end-point of a disinfection procedure on the surfaces in animal houses. In view of the need to standardise testing conditions in the field, the author provides details of the optimum time and place for taking samples under field conditions. Samples for culture media should be taken from the floor when dry. The various techniques (i.e. swabbing, agar cylinder, agar carrier, 'ready-to-use' set) should provide information on the end-point of the disinfection. The desired end-points is approximately one viable bacterium per cm2 of surface area. The technique used should demonstrate whether the disinfection has been effective.