A group of 14 infertile anovulatory women were treated with human menopausal gonadotropin (HMG) when induction of ovulation was not achieved with high doses of clomiphene citrate. The amount of HMG given during each of 43 treatment cycles was determined by measuring estrogen levels by either (a) fluorimetric determination of total urinary estrogens, (b) serum estradiol by radioimmunoassay (RIA), and (c) a rapid 4 hour RIA of immunoreactive serum estrogen. An excellent correlation among each of these methods of assessing estrogen production was found. Employing these methods to determine follicular maturation and adjusting HMG dosage accordingly, ovulation was induced in 38 of the 43 treatment cycles. Pregnancy occurred in 7 of the 14 patients (11 of the 38 ovulatory cycles). Ovarian hyperstimulation occurred in 5 treatment cycles. The results of this study indicate that the RIA of serum immunoreactive estrogen is as accurate to determine follicular growth as the other two methods. Since it is more readily performed the rapid serum immunoreactive estrogen assay appears to be the preferable method of monitoring HMG therapy.