The aim of the present study was to demonstrate induction of vitellogenin in the common carp (Cyprinus carpio) as a biomarker for monitoring freshwater ecosystems. Sexually undifferentiated specimens of common carp were treated experimentally with 17beta-estradiol and increasing doses of 4-nonylphenol and levels of plasma Vtg were measured in order to: 1) validate an ELISA assay for plasma Vtg in the common carp using the polyclonal rabbit anti-salmon Vtg antibody AA1 (Biosense, Norway); 2) check the sensitivity of carp juveniles in producing Vtg in response to estrogen stimulation. The group treated with 17beta-estradiol showed high induction (156%) with respect to controls, also groups treated with 4-nonylphenol showed induction of Vtg. The group treated with 100 mg kg(-1) b.wt showed an induction of 61%. A statistically significant correlation was found between dose and response. This preliminary study demonstrate a response to the ELISA assay for Vtg in the common carp using rabbit anti-salmon antibody AA1. With further evidence the tested biomarker might be proposed for large scale monitoring of estrogenic effects caused by pollution in urban and industrial effluents.