10 - 20 percent of synthetic dyes are discharged without being controlled to the waste waters after the dyeing process. Dyes which are discharged without being controlled reach to the human beings by food chain and threaten their health. In this study, the genotoxicity of synthetic reactive dyes are searched by umu-test (Salmonella thyphimurium TA1535/pSK1002) which is short term bacterial test system. Concentrations of 400 mu g/mL, 120 mu g/mL, 40 mu g/mL and 4 mu g/mL of dye solutions are prepared. By using S9 fraction, the biotransformation effects of the dyes in an organism at the presence of liver enzymes are researched. In this study Reactive Black 5, Reactive Blue 19, Reactive Red 74, Reactive Red 141, Reactive Yellow 84 synthetic dyes are studied. At the end of the study Reactive Yellow 84, Reactive Black 5 have mutagenic effects at the presence of S9 fraction at 400 mu g/mL concentration also Reactive Black 5 has mutagenic effect at the absence of S9 fraction at 400 mu g/mL concentration.