The NSAID in vitro-Bioassay – originally developed for municipal waste water samples – can now be used to detect activity of cox-inhibitors in surface waters as well. By using 44 water samples from several European countries the application range and quality of this bioassay was increased and the limit of detection is further reduced to now 12 ng diclofenac/l. In this project other characteristics like linearity, recovery and accuracy of the test were determined as well. The improved bioassay version can be used as screening tool prior to an extensive chemical analysis. Furthermore, it can be used to avoid false negative assessment and under-estimation of environmental pollutions caused by usage chemical analysis methods only.