In order to elucidate whether specific estrogen-binding sites were heterogeneous, these sites were studied in uterine cytosols of guinea pig and green monkey Cercopithecus griseus. Concentrations of the estrogen-binding sites for labelled estrogens (estrone, estradiol, estriol and diethylstilbestrol) were estimated in the cytosols by means of the exchange method and Scatchard plot analysis. High linear correlation was shown between the Scatchard plots and the curves of estrogen-binding site complexes dissociation plotted in semilogarithmic coordinates (r-0.98 and 0.99, respectively; p less than 0.05). The linearity of the curves was confirmed by means of F-criterion. The data obtained suggest that only one type of specific estrogen-binding sites occurs in the cytosols at 30 degrees and at concentration of estrogens which did not exceed 50 nM. These estrogen-binding sites exhibited the same limited binding capacity for all the estrogens studied.