The genotoxic potential of the waste water of a hospital was evaluated by the umuC test. Within 2 years over 800 native waste water samples were analysed. Genotoxic activity was found in 13% of the samples. The highest genotoxic activity occurred in the morning hours, but genotoxic samples were detected also during the day and at night. 96% of the genotoxic waste water samples revealed a genotoxic potential without growth inhibition of test bacteria monitored as OD600, in the same way as antineoplastic drugs like mitomycin C or cisplatin. 4% of the genotoxic waste water samples showed combined cytotoxic and genotoxic activities as seen in control experiments using glutaraldehyde containing disinfectants and certain antibiotics.