Endocervical gland estrogen receptor (ER) deficiency has been proposed as a possible cause for the poor cervical mucus (CM) in some infertile women with cervical factor. Cytosol ERs were measured in endocervical tissue obtained by biopsy within 3 days of ovulation (determined by the endogenous luteinizing hormone [LH] surge) in five infertile women with persistent poor preovulatory CM (group 1) and in endocervical tissue obtained in the late follicular phase in eight ovulatory women with excellent CM (group 2). ER concentrations were measured in Fmol/mg protein by the dextran-coated charcoal separation method (New England Nuclear Kit, Boston, MA). CM score evaluation and measurement of serum estradiol (E2) and progesterone (P) levels were performed concomitantly. Serum E2 levels of 123.4 +/- 29.3 pg/ml (mean +/- standard error of the mean [SEM]) in group 1 were comparable to E2 levels of 123.3 +/- 15.0 pg/ml in group 2. Serum P concentrations of 0.9 +/- 0.27 ng/ml in group 1 were comparable to 0.79 +/- 0.29 in group 2. A CM score of 4.6 +/- 0.69 in group 1 was significantly lower than 11.6 +/- 0.53 in group 2 (P less than 0.01). The cytosol ER was negative in four of five women in group 1, whereas in group 2, ER was positive in six, borderline in one, and negative in one subject. This study suggests that cytosol ER may be deficient in some women with cervical factor.