This study reports on the development and application of a fish-specific estrogen-responsive reporter gene assay. The assay is based on the rainbow trout (Oncorhynchus mykiss) gonad cell line RTG-2 in which an acute estrogenic response is created by cotransfecting cultures with an expression vector containing rainbow trout estrogen receptor a complementary DNA (rtERalpha cDNA) in the presence of an estrogen-dependent reporter plasmid and an estrogen receptor (ER) agonist. In a further approach, RTG-2 cells were stably transfected with the rtERalpha cDNA expression vector, and clones responsive to 17beta-estradiol (E2) were selected. The estrogenic activity of E2, 17alpha-ethinylestradiol, 4-nonylphenol, nonylphenoxy acetic acid, 4-tert-octylphenol, bisphenol A, o,p'-DDT, p,p'-DDT, o,p'-2,2-bis(chlorophenyl)-1,1-dichloroethylene (o,p'-DDE), p,p'-DDE, o,p'-2,2-bis(chlorophenyl)-1,1-di-chloroethane (o,p'-DDD), p,p'-DDD, and p,p'-2,2-bis(chlorophenyl)acetic acid (p,p'-DDA) was assessed at increasing concentrations. All compounds except o,p'-DDT, p,p'-DDE, and p,p'-DDA showed logistic dose-response curves, which allowed the calculation of lowest-observed-effect concentrations and the concentrations at which half-maximal reporter gene activities were reached. To check whether estrogen-responsive RTG-2 cells may be used to detect the estrogenic activity of environmental samples, an extract from a sewage treatment plant (STP) effluent was assessed and found to have estrogenic activity corresponding to the transcriptional activity elicited by 0.05 nM of E2. Dose-response curves of nonylphenol, octylphenol, bisphenol A, and o,p'-DDD revealed that the RTG-2 reporter gene assay is more sensitive for these compounds when compared to transfection systems recombinant for mammalian ERs. These differences may have an effect on the calculation of E2 equivalents when estrogenic mixtures of known constitution, or environmental samples, such as STP effluents, are assessed.