Steroidal estrogens in the environment are of concern because they exert toxicological effects at very low concentrations. Furthermore, studies indicate dissolved (DOC) and colloidal organic carbon (COC) play an important role in the environmental fate and transport of steroidal estrogens. The aim of this study is to develop and evaluate a method to quantify the toxicological significance of steroidal estrogen detections in the environment. Also, the estrogenic potency of steroid estrogens associated with DOC and COC will be evaluated. Liquid swine manure was filtered to obtain DOC and COC, and radiolabelled estradiol was mixed with the DOC/COC at various concentrations. Fluorescence Polarization was used to measure the response of estrogen receptors to DOC/COC derived from liquid swine manure.  This data was used to develop a toxic equivalence factor for each characterized estrogenic species. Furthermore, field receiving liquid swine manure was instrumented to obtain subsurface water samples. The manure contained approximately 1,800 ng/L 17b-estradiol (E2). Initial subsurface tile drainage samples have been taken from the field location. Field samples will be analyzed to provide background estrogen concentrations and assist in better environmental manure management practices. Anticipated results will determine whether E2 bound to COC/DOC is of concern to organisms in the environment.