To study the mutagenicity of source water with different turbidities, tap water and active carbon-treated tap water, unscheduled DNA synthesis in primary rat hepatocyte was detected. Increased unscheduled DNA synthesis was found in a dose-depend manner inducted by tap water, source water and tap water treated with 5, 10 and 15 milligrammes active carbon per liter water. The results also showed that the mutagenicity of the source water increased with the increasing of its turbidity, and mutagenicity of the tap water is higher than that of the source water as well. There was no unscheduled DNA synthesis in tap water followed by the treatment with 20 milligrammes active carbon per liter water. It is suggested that the development of the water treatment technics including sedimentation, filtration and treatment with active carbon is needed for decreasing or eliminating the mutagenicity of tap water.