Abstract: Synthetic biological toxicity of 54 municipal sludge samples collected from the Yangtze River Delta Region in China was determined with the luminous bacteria method,and HgCl₂,which is relatively stable in toxicity,as reference for quantification.Total acute bio-toxicity of the samples was evaluated by relative light-inhibiting rate of the luminous bacteria,equivalence to concentration of HgCl₂ or EC₅₀.Results show that among the 42 municipal sludge samples that did not need to be diluted before determination,13 or 30.9% were acute in toxicity,and among those that needed diluting for determination,8 or 66.67% were in Grade I in toxicity,suggesting that most of the 54 municipal sludge samples collected from the Yangtze River Delta Region were relatively high in synthetic acute bio-toxicity.It could be related to the high concentrations of heavy metals and toxic organic contaminants in the municipal sludge,and to the type and origin of the municipal sludge and wastewater treatment technology as well.The municipal sludge from Shanghai was higher in synthetic biological toxicity than those from Zhejiang and Jiangsu.Among the municipal sludge samples,relatively low in toxicity,those using the technology of oxidation ditch were mostly higher than those using the technology of A²/O.Most of the sludge samples,relatively high in toxicity,were using the technology of activated sludge and A²/O.Municipal sludge samples from domestic sewage and mixed sewage were mostly higher in toxicity than those from industrial sewage.