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The occurrence and characteristics of an estrogen receptor in the cytosol of myoma samples from human uteri were investigated employing dextran-coated charcoal and density gradient centrifugation techniques. Receptor binding site concentrations in 24 myoma specimens ranged from 23 to 515 fmol/mg cytosol protein (98+/-108, mean+/-S.D.). In one myoma sample no receptor was found. The apparent equilibrium dissociation constant (Kd) was 1.3 X 10(-10) mol/l for estradiol-17beta. On sucrose density gradient centrifugation, [3H]estradiol was bound by macromolecules with sedimentation rates of 4 and 8 S. The latter component was specific for estrogens, whereas the former contained specific and nonspecific binding sites. Ligand specificity studies were carried out utilizing 30 different steroidal compounds. A good correlation was found between the in vitro binding affinity and the in vivo estrogenic potency of the compounds tested. The cytosol estrogen receptor from myoma had a ligand specificity which closely resembled that of the corresponding receptor in normal human myometrium and endometrium as well as in human breast carcinoma. The myoma estrogen receptor level was compared to that in normal myometrium and endometrium in 13 uterine specimens. The receptor concentrations in cytosol fractions from myoma and myometrium correlated significantly (P less than 0.05), whereas no correlation existed between the receptor levels in endometrial and myoma cytosols. Furthermore, the estrogen receptor content in myoma samples did not correlate to estradiol-17beta levels in the myoma cytosol or serum of the same patient.