The protein-interacting ability of estrogen receptor beta (ERbeta) and estrogen receptor alpha (ERalpha) from Atlantic salmon (Salmo salar) was studied using the yeast two-hybrid system. The salmon ERbeta shared features with other nuclear receptors regarding dimerization, forming a homodimer and heterodimer with ERalpha. The activation of salmon ERs by estradiol-17beta was shown to be necessary for dimerization. Furthermore, the xenoestrogens, 4-nonylphenol and bisphenol A, were shown to promote ER dimerization in the yeast two-hybrid system.