he tap waters,raw waters and chlorinated raw waters from the waterworks in Shanghai,Kunming,Harbin,Shengyang, Guangzhou, Wuhan and Shenzhen cities were studied for their characteristics in mutagenicity by using an Ames Test.All the tap waters were found to contain direct mutagens of frameshift, with strain TA98 to be the most sensitive strain for such a monitol'ing.The oh1Grinated raw waters from all the cities except for shenzhen were found to have significantly a stronger mutagenic activity than their raw waters in both strain TA98 and TA100,with and without Sg, as evaluated in terms of relative activity by water volume (rev./L)or by weight of water concentrate(rev./ mg ),particularly in the case of Shanghai.The results provide a scientific basis for selecting representative cities to study the mechanism on the formation of toxic chlorinated organics during chlorination of drinking water.