Purification efficiency is often lowerd when various chemicals are flowed to the activated sludge in wastewater treatment plant. This paper describes the efficiency of various parameters in determining the toxicity of chemicals to activated sludge. The following five parameters were compared by using mercury (II) chloride, mercury (II) acetate, copper (II) chloride and copper (II) acetate as test chemicals ; COD removal rate, oxygen uptake rate (OUR), nitrification, alkaline phosphatase activity (Al-PA) and TTC-succinate dehydrogenase activity (TTC-SDHA), and the following conclusions were obtained ; 1) Among five parameters tested, OUR proved to be the most simple, rapid and sensitive parameter to determine the qualitative toxicity of chemicals to activated sludge. 2) Other four parameters, COD removal rate, nitrification, Al-PA and TTC-SDHA, were fairly time-consuming and less sensitive than OUR.