Abstract: To test sewage for the possible presence of substances which are toxic with respect to sewage biological purification microorganisms, a sample of the sewage is positively provided with a dissolved oxygen content and is then subjected to the action of microorganisms in a biological mass completely submerged in the sample. The volume of the biological mass is controlled to keep it at a constant level. Thereafter the oxygen content of the sample is measured, and if this content is found to be high, it is known that the sample contains toxic substances and that the sewage sample should be treated otherwise than biologically.