An investigation of the effects on follicular development of clomiphene citrate and its two isomers En clomiphene and Zu clomiphene given separately was carried out on 19 normally cycling women being treated with donor insemination. All women received clomiphene citrate in the first cycle and, following a washout control cycle, were treated with either En clomiphene or Zu clomiphene alone. The number of follicles present, follicular phase oestrogen secretion and luteal phase pregnanediol excretion were not significantly different when Zu clomiphene cycles were compared with control cycles, but were significantly increased in En clomiphene and clomiphene citrate cycles. It is concluded that the En isomer, which has largely antioestrogenic properties, is the isomer active in inducing follicular development. The oestrogenic properties of Zu isomer did not appear to protect it from the possibly detrimental effects on sperm-cervical mucus interaction observed in both isomers and in the combined preparation