This document specifies a method for the determination of the estrogenic potential of water and waste water by means of a reporter gene assay with a genetically modified yeast strain Arxula adeninivorans. This reporter gene assay is based on the activation of the human estrogen receptor alpha.Arxula adeninivorans is a highly robust and salt- and temperature-tolerant test organism and is especially suitable for the analysis of samples with high salinity (conductivity up to 70 mS/cm). The test organism can be cultivated in medium with sodium chloride content up to 20 %. This method is applicable to: fresh water; waste water; sea water; brackish 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 1,5 ng/l and 3 ng/l 17²-estradiol equivalents (EEQ). The upper threshold of the dynamic range for this test is between 25 ng/l and 40 ng/l 17²-estradiol equivalents (EEQ). 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. An international interlaboratory trial for the validation of this document has been carried out. The results are summarized in Annex F.