Estrogen-binding proteins were observed in the cytosol of human pituitary adenomas. The Kd for estradiol was 0.44 nM at 0 C, and hormonal specificity was that expected for estrogen receptors. Sucrose gradient sedimentation experiments revealed that receptors from different tumors existed in either the 8S or the 4S form or both. Of the 23 tumors examined, 14 contained estrogen receptors. Receptors were more often present and their concentration was higher in PRL-secreting adenomas (mean +/- SEM, 20.6 +/- 13.4 fmol/mg proteins; 7 determinations) than in GH-secreting adenomas (1.4 +/- 0.8 fmol/mg protein; 9 determinations) and chromophobe tumors (4.1 +/- 1.6 fmol/mg protein; 7 determinations). There was also a correlation between the presence of estrogen receptors and histological signs of cell proliferation and tumor growth.