Penetration of cervical mucus by spermatozoa was observed in cycling and infertile women, in women on sequential or combination oral contraceptive medication, and in women with an IUD in situ. _-Sperm penetration of cervical mucus varied from women not using contraceptives and from infertile women, to patients taking sequential conception control pills. In cycling women penetration was low in the early proliferative phase, high at mid-cycle, and low again in the luteal phase. The infertility patients, studied around mid-cycle, had high penetration. Patients on the sequential pill had moderate to high penetration throughout the cycle. Patients using IUD's had higher than anticipated sperm penetration, especially during the luteal phase of the cycle (p