Dyes which are disposed through uncontrolled discharge reach to human from the environment they are discharged through the food chain and threaten human health. 10-20% of the synthetic dyes which are used in textile industry are discharged to waste water in an uncontrolled manner upon dyeing process. In this study, genotoxic properties of some azo dyes were researched with umu-test (Salmonella thyphimurium TA 1535/pSK1002) which is a short term bacterial test. Solutions of dyes at concentrations of 400 mu g/ml, 120 mu g/ml, 40 mu g/ml and 4 mu g/ml were prepared and biotransformation effects of dyes on organisms were observed in presence of liver enzymes by using S9 fraction. The study analyzed Reactive Black 5, Reactive Blue 19, Reactive Red 141, Reactive Red 120, Reactive Yellow 84, Direct Yellow 86, Direct Blue 200 and Disperse Red 167 azo dyes. In the result of the research mutagenic effect was determined at 400 mu g/ml concentration for Reactive Yellow 84 and Reactive Black 5 in presence of S9 fraction; 400 mu g/ml concentration in absence of S9 fraction of Black 5; 400 mu g/ml concentration and 120 mu g/ml concentration for Direct Blue 200 in presence of S9 fraction; 400 mu g/ml concentration for Direct Blue 200 in absence of S9 fraction.