Reclamation and reuse of wastewater is one of the most effective ways to alleviate water shortage. Disinfection plays a key role in killing the harmful pathogens in reclaimed water, while an unwanted side effect is the formation of disinfection by-products (DBPs). Recently, a number of researches have been conducted on the formation regularities of certain DBPs. However, with current physiochemical techniques, it is impossible to detect all the DBPs. In this study, photobacterium bioassay was used to measure the formation of DBPs and their toxic effect as a whole. The effects of water quality characteristics and operational conditions on the toxicity formation during wastewater chlorination disinfection process were evaluated. A statistical model, depending on chlorine disinfectant dosage, concentration of ammonia nitrogen, and concentration of dissolved organic carbon, was developed to quantitatively estimate the toxicity formation during the disinfection process. It was found that the toxicity of the wastewater samples was positively correlated with chlorine disinfectant dosage, concentration of dissolved organic carbon and UV absorbance at 254 nm, while negatively correlated with concentration of ammonia nitrogen. (c) 2006 Elsevier B.V. All rights reserved.