The effects of ozonation, hydrogen peroxide treatment and chlorination on the quality of organic matter and microbiological growth in drinking water were studied in a pilot plant. All oxidants degraded organic matter (humus) increasing the concentration of assimilable organic carbon (AOC). Even though the oxidants efficiently decreased microbial numbers, microbial regrowth occurred in all drinking waters. Microbial regrowth was strong especially after ozone/hydrogen peroxide treatment. Chlorination combined with ozone and hydrogen peroxide decreased microbial numbers most efficiently. However, microbial growth started even in chlorinated samples within 2-3 days.