Toxicity measurements in activated sludge show that the Michaelis-Menten theory and the theory of enzyme inhibition reveal reproducible data on toxicity and also may reveal, insights into the physiological aspects of toxicity. Thus, it can be shown, that toxicity is complex. It consists of at least two components namely of competitive and noncompetitive inhibition. Some toxic materials tend to have a largely competitive effect, others have a largely noncompetitive effect, resulting either in increase of the Michaelis constant (Km) or in reduction of the maximal activity (Vmax). The effect of the pH is pure noncompetitive inhibition. Sludge age also plays an important role. Increasing sludge age finally results in pure competitive inhibition or noncompetitive inhibition, depending on the nature of the toxic material. This can be proved with copper-sulfate and hexavalent chromium.