This document specifies a method for the determination of the estrogenic potential of water and waste water by means of a reporter gene assay utilizing stably transfected human cells. This method is applicable to: – fresh water; – waste water; – aqueous extracts and leachates; – eluates of sediments (fresh water); – pore water; – aqueous solutions of single substances or of chemical mixtures; – drinking water. – the limit of quantification (LOQ) of this method for the direct analysis of water samples is between 0,3 ng/l and 1 ng/l 17β-estradiol equivalents (EEQ). The upper working range was evaluated [based on the results of the international interlaboratory trial (see Table F.3)] up to a level of 75 ng EEQ/l. Samples showing estrogenic potencies above this threshold have to be diluted for a valid quantification. Extraction and pre concentration of water samples can prove necessary if their estrogenic potential is below the given LOQ.