This International Standard specifies a method for assessing the inhibitory effect of a test material on the oxygen consumption of activated sludge microorganisms. This method is intended to represent the conditions in biological waste-water treatment plants. It gives information on inhibitory or stimulatory effects after a short exposure (usually 30 min up to 180 min or even more) of the test material on activated sludge microorganisms. This method is applicable for testing waters, waste waters, pure chemicals and mixtures of chemicals. Concerning the chemicals, the method refers to those which are soluble under the test conditions. Special care is necessary with materials of low water solubility, high volatility and with materials abiotically consuming or producing oxygen.