An ultrafiltration method employing a Centrifree filter for determining the unbound fraction of estradiol was studied. Centrifugation was performed under conditions similar to those in vivo. Good correlation was recognized between this method and the equilibrium dialysis. This method was employed to determine the unbound fraction of estradiol in the serum and the peritoneal fluid of 26 infertility patients classified according to their menstrual dates. The total estradiol and progesterone contents in the peritoneal fluid were high after ovulation. There was no significant difference in the percentage of unbound estradiol in the serum among various groups. In the peritoneal fluid, however, the percentage of unbound estradiol for the day 12-14 patients was 4.5 +/- 0.2% in contrast with 3.8 +/- 0.4% for the day 15-18 group (p less than 0.05) and 3.5 +/- 0.1% (p less than 0.05) for the day 19-28 group. Moreover, the fraction (4.5%) of unbound estradiol in the peritoneal fluid of a patient with luteinized unruptured follicle (LUF) syndrome was comparable with that of patients in the follicular phase. The difference between the percentage of unbound estradiol in the peritoneal fluid before and after ovulation is considered to be due to the transudation of follicular estradiol in the follicular phase and the exudation of estradiol from the corpus luteum into the peritoneal cavity in the luteal phase.</p
