An enzyme linked immunosorbent assay (ELISA) was developed for the detection of the egg yolk precursor vitellogenin (Vg) in plasma of brown trout (Salmo trutta). Purified Vg from a 17 beta-estradiol-induced trout was used as the competing antigen in the ELISA which is based on polyclonal antibodies. The ELISA's performance was optimized and characterized. The assay's working range was (25-500 ng ml-1), its sensitivity was (10.5 ng ml-1), and it had an intra-assay coefficient of variation of less than 10% between 30 and 1000 ng ml-1. The ELISA was used in bioassays for the detection of environmental estrogens, including estrogen mimics, in whole and fractionated industrial waste waters. Those bioassays were based on intraperitoneal (i.p.) injection-, static renewal-, and flow through exposure systems. The response threshold of both bioassays is limited to 1-2 micrograms ml-1 Vg by a low level plasma interference that was regularly detected in plasma from non-induced male fish. The responsiveness of the bioassays was characterized using progressive doses of 17 beta-estradiol. The i.p.-based assay, which was responsive to at least 100 micrograms kg-1 of 17 beta-estradiol, was used to screen extracts of pulp mill effluent and black liquor for estrogenic effects. Neither extract induced Vg in our assay. The i.p. assay was also used to test 4-tert-octylphenol (OP) and the PAH derivative, retene, for estrogenic activity. OP induced Vg in the i.p.-exposed fish; no Vg induction was detected in the retene-exposed fish. The static renewal bioassay, which was responsive to at least 0.1 microgram ml-1 of 17 beta-estradiol over a 15-day exposure period, was used to screen whole pulp mill effluents for estrogenic effects. No Vg induction was detected in the effluent-treated fish.