The use of ozone as a drinking water disinfectant is increasing. As opposed to the commonly used chlorine the disinfecting activity of ozone cannot be sustained in the water during the distribution process, and as a result recontamination and bacterial growth are not adequately under control. Information about the safety margins of this disinfecting procedure was obtained in experiments simulating the working conditions of a water plant including supply system. After treatment with 1.5 mg O3/1 water, specimens with no detectable remanent ozone were found to remain sterile for at least one month. However, during the passage of a pipeline of 1200 m length recontamination with and considerable growth of microorganisms was observed. After inoculating water sterilized by ozone with a normal population of water bacteria, the bacterial growth was more pronounced than in similar experiments with heat-sterilized water of the same origin. This may be an indication that the breakdown products of organic water contaminants (e.g. humic acid) - resulting from the ozone treatment - are better nutrients for the water bacteria than the original organic substances themselves.