The production of chlorinated by-products through chlorine disinfection of drinking water has been well documented. Natural organic precursors including fulvic and humic acids may produce new mutagenic compounds in their chlorination process. Comparisons of river water and drinking water extracts have confirmed that chlorination during water treatment produces mutagenic activity in the salmonella (Ames) test. The present work concerning XAD-2 and 402 resin extracts of river water from four locations in the Z river and the corresponding chlorinated water from four waterworks has involved two mutagenicity assays for genetic damage:Salmonella test and Syrian hamsters embryo (SHE) cell transformation test. Both river and drinking (chlorinated) water extracts were mutagenic in the Salmonella test and potentially carcinogenic in SHE cell transformation assay. A definite dose response relationship was observed. Compared to the river water samples, the response intensity was higher in the chlorinated samples, The results show that chlorination could increase the genotoxic intensity of drinking water extracts. The results obtained from bacterial mutagenic assay tend to be consistent with those results of SHE cell transformation assay. The production of chlorinated by-products through chlorine disinfection of drinking water has been well documented. Natural organic precursors including fulvic and humic acids may produce new mutagenic compounds in their chlorination process. Comparisons of river water and drinking water extracts have confirmed that chlorination during water treatment of mutagenic activity in the salmonella (Ames) test. The present work concerning XAD-2 and 402 resin extracts of river water from four locations in the Z river and the corresponding chlorinated water from four waterworks has involved two mutagenicity assays for genetic damage: Both river and drinking (chlorinated) water extracts were mutagenic in the Salmonella test and potentially carcinogenic in SHE cell transformation assay. Compared to the river water samples, the response intensity was higher in the chlo rinated samples, The results show that chlorination could increase the genotoxic intensity of drinking water extracts. The results obtained from bacterial mutagenic assay tend to be consistent with those results of SHE cell transformation assay.