We have estimated the binding characteristics of an oestrogen binding protein in the human vagina and in the human myometrium. The specificities of these binding proteins were analyzed in binding studies using competitors and ligands with different structural features representing the various elements considered to be of importance for binding. The specificity of an oestrogen binding protein in human myometrium was found to be similar to those of oestrogen binding proteins in human breast tumour tissue and in human MCF-7 cells. This specificity was also similar to those of oestrogen receptors in myometrium, endometrium and vagina of the rat. However, the specificity of an oestrogen binding protein in the vagina of postmenopausal patients was different. The vaginal oestrogen-binding protein displayed similar high affinities for 17 beta-oestradiol, 17 alpha-oestradiol and oestriol but low affinity for diethylstilbestrol. The equilibrium dissociation constants for [3H]oestriol and [3H]17 beta-oestradiol were 4.3 X 10(-10) M and 4.0 X 10(-10) M respectively and the concentration of this protein varied between 30 and 170 fmol/mg vaginal cytosol protein. We conclude that the human vagina contains an oestrogen binding protein with characteristics different from those of oestrogen receptors present in myometrium and breast tumour tissue.