This International Standard specifies a method for assessing the potential toxicity of a test material to the growth of aerobic bacteria present in activated sludge. The inhibitory effect is restricted to those micro-organisms capable of growth on the chosen organic test medium. This method gives information on inhibitory effect on the microorganisms over incubation periods up to 6 h. This method is applicable to water, wastewater and chemical substances which are soluble under the conditions of the test. Special care is needed with volatile or coloured materials and materials which form turbid suspensions or dispersions.