A reversed-phase fractionation method with subsequent biological and chemical analysis has been developed to estimate the contributions of the most potent estrogens to observed estrogenic effect potentials. Surface water samples were taken in the German Baltic Sea (Inner Wismar Bay and Darss Peninsula, sampling campaign July 2003) and were separated into seven individual fractions. Three fractions showed significant estrogenic activities and clear dose-dependant responses were obtained in the yeast estrogen screen (YES). In the 2nd fractions liquid chromatographic-electrospray-tandem mass spectrometric (LC-ESI-MS-MS) analyses showed the presence of bisphenol A (Inner Wismar Bay: 4.8 ng L ^-1 and 6 ng L ^-1; Darss Peninsula: 0.91 ng L ^-1 and 1.7 ng L ^-1) and ethinylestradiol (Inner Wismar Bay: 2.0 ng L ^-1 and 6.0 ng L ^-1; Darss Peninsula: < MDL and 1.7 ng L ^-1), whereas estrogenic activities in the YES were only around 10% of the positive control E2. Although not identified prior in the total extract the natural hormones estradiol (Inner Wismar Bay: 0.13 ng L ^-1 and 0.19 ng L ^-1; Darss Peninsula: 0.12 ng L ^-1 and 0.16 ng L ^-1) and estriol (Inner Wismar Bay: < MDL and 0.33 ng L ^-1; Darss Peninsula: < MDL) could be detected in the 3rd fractions, where high estrogenic potentials could be observed. The 4th fractions showed high responses as well and estrone were herein quantified with concentrations of 0.16 ng L ^-1 and 0.18 ng L ^-1 (Darss Peninsula) up to 0.37 ng L ^-1 (Inner Wismar Bay). Measured and calculated estradiol equivalents for individual fractions correlated very well (R ² = 0.78), when disregarding results of the 2nd fraction, where high deviations occurred.