The adsorption of trace contaminants on natural particles is of great interest as this adsorption enhances the possibility of removing these with low pressure filtration such as MF and UF. This study shows that natural hormones such as estrone and estradiol will adsorb to natural particles both organic and inorganic. Experiments have shown that the adsorption capacity of organics is a factor of 10 higher than for inorganics. Ultrafiltration of a solution containing NOM and estrone also indicated this adsorption as the retention of estrone showed strong relationship to the concentration of organic material in the permeate. For practical application of this finding it was investigated if it applied to activated sludge. It was found that activated sludge does adsorb the natural hormones and that activated sludge during ultrafiltration develops a dynamic membrane which enhances the retention of the hormones.