AbstractFor a correct determination of the activated sludge activity with the aid of TTC an optimum TTC concentration is required not only in the solution, but also as related to the biomass. Since TTC has toxic effects on bacteria, the selection of the optimum TTC concentration is connected with problems: in the suboptimal range the TTC concentration as referred to the existing bioactivity is too low, in the supraoptimal range the toxic effect does not permit any correct determination of activity. A generally optimum range cannot be stated due to the very different activities and sensitivities of the present bacterial populations. Therefore, the application of the TTC methods requires the simultaneous parallel application of different TTC concentrations and the determination of the activity in the plateau range of the relation between the TTC concentration and the formazan concentration formed.