For the Saccharomyces cerevisiae test the procedures described by Zimmermann et al.12 and Stehrer-Schmid23 were used. Colonies of strains D7 were grown to saturation according to Zimmermann12 and stored at 4°C. During storing phase spontaneous reversion rates were determined. Cultures with the lowest spontaneous background (30–60 convertants/105 cells and 10–20 revertants/106 cells) were grown at 28 °C to exponential phase of growth (6–8×107 cells/mL) and washed cells were suspended in 125mM phosphate buffer pH = 7.2. Exponential cultures were used, because these cells responded with higher sensitivity to mutagenic/carcinogenic action, according to Zimmermann.12 Cells were exposed to drinking water samples for 4h at 28 °C and washed cells were plated on appropriate media to detect revertants, convertants and survival rate. The controls followed the same experimental protocol and were prepared with cells treated with 0.1 and 0.01 mM ethil-methane-sulphonate without and with metabolic bioactivation (–S9 and +S9). Five plates in each category were incubated at 28 °C for 3 days for survival rates and frequency of convertants determination and 6–8 days for frequency of revertant determination. In every test the actual colony count were related to 105 for convertants and revertants cells surviving the treatment with the sample