There is increasing concern about man-made chemicals in the aquatic environment that mimic the action of the steroid hormone, 17b-oestradiol. One of the most sensitive responses to the xenobiotics oestrogens in fish is the induction of the egg-yolk precursor vitellogenin which is normally synthetised by female under oestrogen stimulation. In immature or male fish, vitellogenin may thus serve as a reliable biomarker for xenobiotic oestrogen. In this study, a competitive enzyme linked immunosorbent assay (ELISA) using anti-salmon vitellogenin antibodies (Biosense Laboratories) was developed for the quantification of plasma vitellogenin in rainbow trout. Cross-reactivity and specificity of the antibodies with rainbow trout Vitellogenin was assessed by western blotting and ELISA and no cross-reactivity was observed with juvenile plasma. Vitellogenin was purified from plasma of oestradiol-treated juvenile fish by anion exchange chromatography followed by gel filtration. Purity was confirmed by silver staining Native PAGE, and hence the purified vitellogenin was used as standard in the ELISA. The sensitivity range of the ELISA was 16-175 ng/ml and the detection limit was 7,5 ng/ml. The repetability and the reproductibility were 5-9,5% and