The anti-oestrogens, clomiphene citrate, tamoxifen and cyclofenil are commonly used in the treatment of female infertility. Their role in the management of anovulation is well established but there is continuing controversy about their relevance to other areas of management. We have studied the effects of each of these drugs on cervical mucus and sperm-cervical mucus interaction among 23 patients with unexplained infertility. Each patient received all three drugs in an alternative month treatment regime and in addition acted as her own control. The starting point in each patient was randomized. Luteinizing hormone (LH) and oestradiol were measured daily from day 10, and follicle scanning was also undertaken. Cervical mucus quality and sperm - cervical mucus interaction were studied on the day of onset of the LH surge. The use of clomiphene and tamoxifen resulted in a significant reduction in cervical mucus score and sperm-cervical mucus interaction as judged by the distance travelled by the vanguard spermatozoa. Cyclofenil had no effect on these parameters.