We studied two groups of females to investigate the effect of endogenous oestradiol levels on total and free protein S (tPS, fPS) plasma concentrations. One group (group I) consisted of 12 healthy volunteers who were studied throughout one menstrual cycle; the other group (group II) consisted of 16 young women who were treated with GnRH analogues and gonadotropins before undergoing in vitro fertilization. Neither tPS nor fPS varied significantly with respect to the physiological changes of oestradiol or to the very low and high levels of oestradiol, achieved after GnRH analogues suppression and gonadotropin stimulation. These results indicate that endogenous oestradiol does not affect PS concentration.